Here is a pretty good analogous explanation which shows that according to natural law homosexuality is a sin or wrong.
From: mncuf.org:
By the force of human intellect and by revealed truth it is easy enough to determine that the practice of homosexuality is wrong and evil.
The ability of the human mind to figure things out on its own based on observation is known as “reason”. Human beings can determine “by reason the good and the evil, the truth from the lie. . . .” (CCC1954) By reason we can determine a sort of “natural law” which agrees with revealed truth. “The natural law, present in the heart of each man, and established by reason, is universal in its precepts and its authority extends to all men.”(CCC1956)
We know by reason that the natural law tells us that, for example, food is for sustenance. If one were to decide that buying food, cooking it and eating it, are a waste of time and money, and therefore he will not eat, such a person would starve to death. We need food to live. This is the law of nature, and of God Who made nature. Now suppose someone were to decide that they were going to eat for pleasure alone and, not wanting to experience the natural result of such activity, deliberately induce vomiting to keep from gaining weight. Such an activity is not ordered towards the natural process of eating and digestion. This fact is recognized by mental health professionals and known as an eating disorder called bulimia. In this instance the secular community agrees with the Church’s teaching. Such action is obviously disordered. Food is meant to go on a one-way trip. The organs of digestion are designed for this process.
In the same way the sexual organs are designed for certain functions. God made us male and female, “...male and female he created them.”(GEN 1:27) The male and female bodies, to state an obvious fact, are different. By light of reason alone we can tell that the male and female organs are for made for different purposes. We can also determine by reason alone what those purposes are. When someone uses his or her sexual organs for purposes other than those for which they are specifically designed, such actions are disordered. Just as with bulimia, “...‘homosexual acts are intrinsically disordered.’ They are contrary to the natural law.”(CCC2357) It is infinitely more disordered to use our sexual organs for uses other than their intended purpose than it is to deliberately throw up our supper. It does not matter if one enjoys throwing up, or has real feelings for someone of the same gender. It is not the way God wants us to be. We can tell by the bodies He gave us.
Just as many who have an eating disorder claim there is nothing wrong with them, many who are afflicted with a sexual disorder claim there is nothing wrong with them. Lately they have claimed that homosexuals are born that way and therefore it is natural. This is not true. There are speech therapists in San Francisco that earn hundreds of dollars an hour teaching homosexual men to speak in an effeminate manner. Bull lesbians go out of their way to act like men, with short men’s haircuts and tough masculine clothing. If all their claims about being born that way are true why do they specifically, consciously imitate the opposite sex? By reason alone we can see something is fishy.
The secular world agreed that such activity is disordered until recently. The American Psychiatric Association (APA) had studied the causes of homosexuality for years and had listed it as a mental illness.1 The open study on the matter ended in 1970 when militant homosexuals broke into the APA’s yearly meeting and by screaming obscenities, physical intimidation, and veiled threats forced the members into appointing a special panel on the subject. Their tactics sounded ominously familiar in that one of their demands was “dialogue”.2 After that the militants stacked the panel, and intimidated the main body of the annual meetings until the APA declared in 1973 that homosexuality was no longer considered a mental illness. By reason alone we know that threatening a bunch of weak-willed doctors does not change the truth.
As far as revealed truth, Scripture speaks clearly and forcefully on the subject. Look up (ROM 1:6) (1COR 6:10) (1TIM 1:10) (JUD 1:6-7). There is no doubt that homosexual activity is contrary to the will of God.
It is true we are all sinners. It is true we must not claim or pretend to be better than anyone else. We need to deal with those afflicted with disorders with charity. Mankind’s intellect is darkened by disorders. It is also true that the natural law is for all mankind, and for all times. “The natural law is immutable and permanent throughout the variations of history.” (CCC 1958) God’s law is eternal, it never changes. We must never quit telling the truth about God’s eternal law, even if some people don’t want to hear it.
1 William Dannemeyer, Shadow in the Land, Homosexuality in America (San Francisco 1989) Ignatius Press, 24-39
2 Ibid., 30
Here is scripture which supports the fact that homosexuality is a sin:
Gen. 1 & 2 - we see from the beginning that the complimentarity of the sexes reflects God's inner unity and His creative power and Fatherhood. God created man and woman to become one flesh which is consummated in the act of marital love.
Gen. 2:18 – throughout the creation story, God says “it is good” seven times. But when God pointed out that man was alone, God says “it is not good.” God then created woman. Man and woman therefore belong together by God’s design, according to His natural and supernatural law.
Gen. 2:24 – God created man and woman so that they could share communion. This communion is consummated in the marital act (which must be between a man and a woman). This communion is also a reflection of the eternal communion of the Blessed Trinity, who created man in His own image and likeness.
Gen. 19:24-28 - the Lord rained fire and brimstone on Sodom and Gomorrah as punishment for the sin of homosexuality. Homosexuality perverts God’s covenant with humanity.
Gen 13:13; 18:20: while homosexuals argue that the sin in Genesis 19 was the sin of “inhospitality,” these verses show that the Sodomites were evil and wicked people before the alleged sin of inhospitality. That is because the Sodomites were guilty of homosexuality, not inhospitality. In fact, 2 Pet 2:6:8 says that Lot witnessed their evil sexual practices “day after day.” The Sodomites’ sin was daily and pervasive.
Gen 19:5: this confirms that the Sodomites’ sin is homosexuality. Here, the Sodomites mistaken the two angels for men and, surrounding Lot’s house, ask to have sex with them. When the yell to Lot “bring them out that we may know them,” the Hebrew for “to know” (yadah) in Gen 19:5 means having sexual relations. See, for example, Gen 4:25 where “yadah” refers to sexual relations between Adam and Eve.
2 Pet 2:6-10: Peter also proves that the Sodomites were guilty of homosexuality. He says God condemned them for “licentiousness” and “lust of defiling passion,” which is where we get the title “Sodomite” for a homosexual. The Greek “aselgeia anastrophes” for lustful behavior is also found in Rom 13:13; Gal 5:19; Eph 4:19 and Jude 4.
Jude 7: Jude also confirms the Sodomites’ sin is homosexuality. Jude says they “indulged in unnatural lust” (Greek, ekporneusasai) and went after “strange flesh” (Greek, sarkos heteras) in reference to their homosexual sins.
2 Peter 2:4-6: Peter also reveals that the Sodomites are suffering in hell for their sins by comparing the Sodomites’ punishment to the eternal punishment of the evil angels. Just as God did not spare the angels when they sinned but cast them into hell, so He did the same with the Sodomites when He “condemned them to extinction and made them an example to those who were to be ungodly.”
Jude 5-7: Like Peter, Jude confirms that the Sodomites are in hell by saying that they had the same fate as the evil angels. Jude says just as the evil angels were condemned to hell, the Sodomites “likewise” (Greek, hos) acted immorally and are undergoing a punishment of eternal fire. “Undergoing” (Greek, upechousai) is a present participle which means their suffering is ongoing. See also the clear polarity between those who are saved (v.5) and those who are condemned (vv. 6-7,11,13).
Lev. 18:22, 29 - God commands a man never to lie with a male as with a female, or he will be cut off. This refers to supernatural death which is eternal separation from God. While the Old Covenant laws have been abrogated by the New Covenant, the Church has adopted their moral principles. Thus, the moral prohibitions on homosexuality still apply in the New Covenant, as Peter, Paul and Jude demonstrate.
Lev. 20:13 - God says that if a man lies with another man, he shall be put to death. From the very beginning, God has revealed that homosexual acts are intrinsically disordered, unhealthy, and mortally sinful.
Deut. 22:5 - cross-dressing is also considered an abomination before God.
Matt. 19:6 - after referring to God's divine plan for man and woman, Jesus says a husband and wife become one flesh, which ultimately reflects God's union with humanity through the Church. Homosexual unions pervert this divine truth of God’s love for and union with the human race.
Rom. 1:26 - also, when a woman lies with another women (lesbianism), this is unnatural and a perversion. God wants His children to be pure and holy as He is holy. Paul condemns all homosexual acts, whether committed by men or women.
Rom. 1:27 – Paul calls the practice of homosexuality shameless, unnatural and a perversity. It is contrary to the natural law, as it eviscerates the life-giving aspect of human sexuality and reduces it to a selfish, pleasure-seeking end.
1 Cor 6:9: Paul teaches that sodomites will not enter into the kingdom of God. The Greek word for “sodomite” (arsenokoites) literally means “male copulation” or “male sexual relations.” The Greek “arsen” means “male” and “koites” means “coitus” or “sexual relations.” Paul uses the same word in 1 Tim 1:10.
1 Tim. 1:10 - sodomites are called ungodly and sinners, unholy and profane, lawless and disobedient. They are called by God to chastity. It is important to note that homosexual attractions and inclinations, while disordered and dangerous, are not by themselves sinful per se. It is the acting out on homosexual attraction that is sinful. Those with homosexual desires can still live a life worthy of Christ by remaining chaste and pure as they abstain from acting out on their desires.
Further proof here from ScriptureCatholic.com:
"You shall not commit fornication; you shall not commit adultery; you shall not be a corrupter of youth." Letter of Barnabas 10 (A.D. 74).
"You shall not be a corrupter of boys, nor like unto such." Letter of Barnabas 10 (A.D. 74).
"You shall not commit murder, you shall not commit adultery, you shall not commit pederasty, you shall not commit fornication, you shall not steal, you shall not practice magic, you shall not practice witchcraft, you shall not murder a child by abortion nor kill one that has been born." Didache 2:2 (A.D. 90).
"[W]e have been taught that to expose newly-born children is the part of wicked men; and this we have been taught lest we should do anyone harm and lest we should sin against God, first, because we see that almost all so exposed (not only the girls, but also the males) are brought up to prostitution. And for this pollution a multitude of females and hermaphrodites, and those who commit unmentionable iniquities, are found in every nation. And you receive the hire of these, and duty and taxes from them, whom you ought to exterminate from your realm. And any one who uses such persons, besides the godless and infamous and impure intercourse, may possibly be having intercourse with his own child, or relative, or brother. And there are some who prostitute even their own children and wives, and some are openly mutilated for the purpose of sodomy; and they refer these mysteries to the mother of the gods." Justin Martyr, First Apology 27 (A.D. 151).
"All honor to that king of the Scythians, whoever Anacharsis was, who shot with an arrow one of his subjects who imitated among the Scythians the mystery of the mother of the gods . . . condemning him as having become effeminate among the Greeks, and a teacher of the disease of effeminacy to the rest of the Scythians." Clement of Alexandria, Exhortation to the Greeks 2 (A.D. 190).
"[According to Greek myth] Baubo [a female native of Elusis] having received [the goddess] Demeter hospitably, reached to her a refreshing draught; and on her refusing it, not having any inclination to drink (for she was very sad), and Baubo having become annoyed, thinking herself slighted, uncovered her shame, and exhibited her nudity to the goddess. Demeter is delighted with the sight--pleased, I repeat, at the spectacle. These are the secret mysteries of the Athenians; these Orpheus records." Clement of Alexandria, Exhortation to the Greeks 2 (A.D. 190).
"It is not, then, without reason that the poets call him [Hercules] a cruel wretch and a nefarious scoundrel. It were tedious to recount his adulteries of all sorts, and debauching of boys. For your gods did not even abstain from boys, one having loved Hylas, another Hyacinthus, another Pelops, another Chrysippus, another Ganymede. Let such gods as these be worshipped by your wives, and let them pray that their husbands be such as these--so temperate; that, emulating them in the same practices, they may be like the gods. Such gods let your boys be trained to worship, that they may grow up to be men with the accursed likeness of fornication on them received from the gods." Clement of Alexandria, Exhortation to the Greeks 2 (A.D. 190).
"[A]ll other frenzies of the lusts which exceed the laws of nature, and are impious toward both [human] bodies and the sexes, we banish, not only from the threshold but also from all shelter of the Church, for they are not sins so much as monstrosities." Tertullian, Modesty 4 (A.D. 220).
"[God forbid the Jews to eat certain foods for symbolic reasons:] For that in fishes the roughness of scales is regarded as constituting their cleanness; rough, and rugged, and unpolished, and substantial, and grave manners are approved in men; while those that are without scales are unclean, because trifling, and fickle, and faithless, and effeminate manners are disapproved. Moreover, what does the Law mean when it…forbids the swine to be taken for food? It assuredly reproves a life filthy and dirty, and delighting in the garbage of vice…Or when it forbids the hare? It rebukes men deformed into women." Novatian, The Jewish Foods 3 (A.D. 250).
"[T]urn your looks to the abominations, not less to be deplored, of another kind of spectacle…Men are emasculated, and all the pride and vigor of their sex is effeminated in the disgrace of their enervated body; and he is more pleasing there who has most completely broken down the man into the woman. He grows into praise by virtue of his crime; and the more he is degraded, the more skillful he is considered to be. Such a one is looked upon--oh shame!--and looked upon with pleasure…nor is there wanting authority for the enticing abomination…that Jupiter of theirs [is] not more supreme in dominion than in vice, inflamed with earthly love in the midst of his own thunders…now breaking forth by the help of birds to violate the purity of boys. And now put the question: Can he who looks upon such things be healthy-minded or modest? Men imitate the gods whom they adore, and to such miserable beings their crimes become their religion." Cyprian of Carthage, Letters 1:8 (A.D. 253).
"Oh, if placed on that lofty watch-tower, you could gaze into the secret places--if you could open the closed doors of sleeping chambers and recall their dark recesses to the perception of sight--you would behold things done by immodest persons which no chaste eye could look upon; you would see what even to see is a crime; you would see what people embruted with the madness of vice deny that they have done, and yet hasten to do--men with frenzied lusts rushing upon men, doing things which afford no gratification even to those who do them." Cyprian of Carthage, Letters 1:9 (A.D. 253).
"[T]he mother of the gods loved [the boy Attis] exceedingly, because he was of most surpassing beauty; and Acdestis [the son of Jupiter] who was his companion, as he grew up fondling him, and bound to him by wicked compliance with his lust…Afterwards, under the influence of wine, he [Attis] admits that he is…loved by Acdestis…Then Midas, king of Pessinus, wishing to withdraw the youth from so disgraceful an intimacy, resolves to give him his own daughter in marriage…Acdestis, bursting with rage because of the boy's being torn from himself and brought to seek a wife, fills all the guests with frenzied madness; the Phrygians shriek, panic-stricken at the appearance of the gods . . . [Attis] too, now filled with furious passion, raving frantically and tossed about, throws himself down at last, and under a pine tree mutilates himself, saying, `Take these, Acdestis, for which you have stirred up so great and terribly perilous commotions.'" Arnobius, Against the Pagans 5:6-7 (A.D. 305).
"[H]aving forbidden all unlawful marriage, and all unseemly practice, and the union of women with women and men with men, he [God] adds: `Do not defile yourselves with any of these things; for in all these things the nations were defiled, which I will drive out before you. And the land was polluted, and I have recompensed [their] iniquity upon it, and the land is grieved with them that dwell upon it' [Lev. 18:24-25]." Eusebius of Caesarea, Proof of the Gospel 4:10 (A.D. 319).
"He who is guilty of unseemliness with males will be under discipline for the same time as adulterers." Basil the Great, Letters 217:62 (A.D. 367).
"If you [O, monk] are young in either body or mind, shun the companionship of other young men and avoid them as you would a flame. For through them the enemy has kindled the desires of many and then handed them over to eternal fire, hurling them into the vile pit of the five cities under the pretense of spiritual love. At meals take a seat far from other young men. In lying down to sleep let not their clothes be near yours, but rather have an old man between you. When a young man converses with you, or sings psalms facing you, answer him with eyes cast down, lest perhaps by gazing at his face you receive a seed of desire sown by the enemy and reap sheaves of corruption and ruin. Whether in the house or in a place where there is no one to see your actions, be not found in his company under the pretense either of studying the divine oracles or of any other business whatsoever, however necessary." Basil the Great, The Renunciation of the World (A.D. 373).
"[The pagans] were addicted to the love of boys, and one of their wise men made a law that pederasty…should not be allowed to slaves, as if it was an honorable thing; and they had houses for this purpose, in which it was openly practiced. And if all that was done among them was related, it would be seen that they openly outraged nature, and there was none to restrain them… As for their passion for boys, whom they called their 'paedica,' it is not fit to be named." John Chrysostom, Homilies on Titus 5 (A.D. 390]).
"[Certain men in church] come in gazing about at the beauty of women; others curious about the blooming youth of boys. After this, do you not marvel that [lightning] bolts are not launched [from heaven], and all these things are not plucked up from their foundations? For worthy both of thunderbolts and hell are the things that are done; but God, who is long-suffering, and of great mercy, forbears awhile his wrath, calling you to repentance and amendment." John Chrysostom, Homilies on Matthew 3:3 (A.D. 391).
"All of these affections [in Rom. 1:26-27]… were vile, but chiefly the mad lust after males; for the soul is more the sufferer in sins, and more dishonored than the body in diseases." John Chrysostom, Homilies on Romans 4 (A.D. 391).
"[The men] have done an insult to nature itself. And a yet more disgraceful thing than these is it, when even the women seek after these intercourses, who ought to have more shame than men." John Chrysostom, Homilies on Romans 4 (A.D. 391).
"And sundry other books of the philosophers one may see full of this disease. But we do not therefore say that the thing was made lawful, but that they who received this law were pitiable, and objects for many tears. For these are treated in the same way as women that play the whore. Or rather their plight is more miserable. For in the case of the one the intercourse, even if lawless, is yet according to nature; but this is contrary both to law and nature. For even if there were no hell, and no punishment had been threatened, this would be worse than any punishment." John Chrysostom, Homilies on Romans 4 (A.D. 391).
"[T]hose shameful acts against nature, such as were committed in Sodom, ought everywhere and always to be detested and punished. If all nations were to do such things, they would be held guilty of the same crime by the law of God, which has not made men so that they should use one another in this way." Augustine, Confessions 3:8:15 (A.D. 400).
"[Christians] abhor all unlawful mixtures, and that which is practiced by some contrary to nature, as wicked and impious." Apostolic Constitutions 6:11 (A.D. 400).
