I  The Catholic teaching of this matter is given to us by God Himself in Sacred Scripture:

Genesis XIII, 13:  "And the men of Sodom were very wicked, sinners before the face of the Lord, beyond measure"; these Sodomites were in fact homosexuals, as it is proven by Genesis XIX 1-11.

Leviticus 18:22,24-30:  "Thou shall not lie with mankind as with womankind, because it is an abomination.  (...)  Defile not yourselves with any of these things with which all the nations have been defiled, which I will cast out before you.  And with which the land is defiled: the abominations of which I will visit, that it may vomit out its inhabitants.  Keep ye my ordinances and my judgments, and do not any of these abominations: neither any of your own nation, nor any stranger that sojourneth among you.  For all these detestable things the inhabitants of the land have done, that were before you, and have defiled it.  Beware then, lest in like manner, it vomit you also out, if you do the like things, as it vomited out the nation that was before you.  Every soul that shall commit any of these abominations, shall perish from the midst of his people.  Keep my commandments.  Do not the things which they have done, that have been before you, and be not defiled therein.  I am the Lord your God".

Leviticus 20:13:  "If any one lie with a man as with a woman, both have committed an abomination, let them be put to death: their blood be upon them".

Deuteronomy 23:18:  "Thou shall not offer the hire of a strumpet, nor the price of a dog, in the house of the Lord thy God, whatsoever be that thou hast vowed: because both these are an abomination to the Lord thy God".

1 Kings 14:24, "And there were also sodomites in the land: and they did according to all the abominations of the nations which the LORD cast out before the children of Israel."

1 Kings 15:11-12, "And Asa did that which was right in the eyes of the LORD, as did David his father. And he took away the sodomites out of the land, and removed all the idols that his fathers had made."

Romans 1: 26-27, 31-32:  "For this cause God delivered them up to shameful afflictions.  For their women have changed the natural use into that use which is against nature.  And in like manner the men also, leaving the natural use of the women, have turned in their lusts one towards another, men with men working that which is filthy, and receiving in themselves the recompense which was due to their error.  (...)  Foolish, dissolute, without affection, without fidelity, without mercy.  Who, having known the justice of God, did not understand that they, who do such things, are worthy of death: and not only they that do them, but they also that consent to them that do them.

Jude 1:7 -“As Sodom and Gomorrah, and the neighboring cities, in like manner, having given themselves to fornication, and going after strange flesh, were made an example, suffering the punishment of eternal fire.”

I Corinthians VI, 9-10:  "Know you not that the unjust shall not possess the kingdom of God? Do not err: Neither fornicators, nor idolaters, nor adulterers.  Nor the effeminate, nor liars with mankind, (...) shall possess the kingdom of God".  See Genesis 19:12-29. Consequently, St. Augustine says (1) "that of all these - namely the sins belonging to lust - that which is against nature is the worst," is ”in itself a mortal sin," (2) a sin specifically distinct (3) from all other sins.

II  Why is homosexuality an "abomination", excluding from the heavenly kingdom?

A.  Because homosexuality is against nature: In the natural order planned by God, the purpose of the carnal union is generation.  Now, generation is the natural fruit of the carnal union of man and woman.  Therefore the carnal union of two persons of the same sex goes against the natural order planned by God.

B.  Because homosexuality is against reason: Reason presupposes things as determined by nature, before disposing of other things according as it is fitting.  Now, in matters of action, it is most grave and shameful to act against things as determined by nature.  Therefore, since the homosexual transgresses that which has been determined by nature with regards to the use of sexual actions, it follows that he acts against reason.

III  Thoughts of Fathers and Doctors of the Church about homosexuality:

St. Augustine

 The great Bishop of Hippo writes: “Sins against nature, therefore, like the sin of Sodom, are abominable and deserve punishment whenever and wherever they are committed. If all nations committed them, all alike would be held guilty of the same charge in God’s law, for our Maker did not prescribe that we should use each other in this way. In fact, the relationship that we ought to have with God is itself violated when our nature, of which He is Author, is desecrated by perverted lust.” Further on he reiterates: “Your punishments are for sins which men commit against themselves, because, although they sin against You, they do wrong in their own souls and their malice is self-betrayed. They corrupt and pervert their own nature, which You made and for which You shaped the rules, either by making wrong use of the things which You allow, or by becoming inflamed with passion to make unnatural use of things which You do not allow” (Rom. 1:26). (St. Augustine, Confessions, Book III, chap. 8)

St. John Chrysostom

"The worst of it is that such an abomination is committed boldly and that the monstrosity becomes the law.  Nobody nowadays fears, nobody blushes.  They boast and they laugh at these actions.  The people who abstain appear stupid and they who condemn are regarded as fools.  If they appear to be the weaker ones they are crushed with blows.  If they are stronger, people laugh, people mock them and make many jokes about them.  They have no redress in tribunals or in law."  (...) "I have heard also many men who are surprised that up to the present a new shower of fire has not fallen on us and that the chastisement of Sodom has not fallen again on our town which is even more deserving of punishment since it did not learn from the evils of the Sodomites.  Although after two thousand years this place accursed and overwhelmed which was Sodom cries to the whole world by its appearance more eloquently than any one voice could, not to dare to commit such heinous offences, our fellow citizens have committed these offences not with less effrontery but quite on the contrary they show themselves more daring and unashamed as if they were determined to do battle with God and that they wish to prove that they wish to add to their crimes, in proportion as the threats become more terrible.  How is it that according as the crimes of Sodom renew themselves the chastisement of Sodom is not also renewed?  Ah, the reason is that a more terrible fire waits them and that a chastisement is reserved for them which will have no end." (Against the opponents of Monastic Life, III.8)

Furthermore commenting on the Epistle to the Romans (1: 26-27), he says that the pleasures of sodomy are an unpardonable offense to nature and are doubly destructive, since they threaten the species by deviating the sexual organs away from their primary procreative end and they sow disharmony between men and women, who no longer are inclined by physical desire to live together in peace. The brilliant Patriarch of Constantinople employs most severe words for the vice we are analyzing. Saint John Chrysostom makes this strong argument: “All passions are dishonourable, for the soul is even more prejudiced and degraded by sin than is the body by disease; but the worst of all passions is lust between men…. The sins against nature are more difficult and less rewarding, since true pleasure is only the one according to nature. But when God abandons a man, everything is turned upside down! Therefore, not only are their passions [of the homosexuals] satanic, but their lives are diabolic….. So I say to you that these are even worse than murderers, and that it would be better to die than to live in such dishonour. A murderer only separates the soul from the body, whereas these destroy the soul inside the body….. There is nothing, absolutely nothing more mad or damaging than this perversity.” (St. John Chrysostom, In Epistulam ad Romanos IV, in J. McNeill, op. cit., pp. 89-90)
 

Tertullian

The great apologist of the Church in the second century, writes: “All other frenzies of lusts which exceed the laws of nature and are impious toward both bodies and the sexes we banish … from all shelter of the Church, for they are not sins so much as monstrosities.” (Tertullian, De pudicitia, IV, in J. McNeil, op. cit., p. 89)

Saint Gregory the Great.

This great saint delves deeper into the symbolism of the fire and brimstone that God used to punish the sodomites: “Brimstone calls to mind the foul odors of the flesh, as Sacred Scripture itself confirms when it speaks of the rain of fire and brimstone poured by the Lord upon Sodom. He had decided to punish in it the crimes of the flesh, and the very type of punishment emphasized the shame of that crime, since brimstone exhales stench and fire burns. It was, therefore, just that the sodomites, burning with perverse desires that originated from the foul odor of flesh, should perish at the same time by fire and brimstone so that through this just chastisement they might realize the evil perpetrated under the impulse of a perverse desire.” (St. Gregory the Great, Commento morale a Giobbe, XIV, 23, vol. II, p. 371, Ibid., p. 7)

Saint Basil of Caesarea

This fourth century Church Father who wrote the principal rule of the monks of the East, establishes this rule: “The cleric or monk who molests youths or boys or is caught kissing or committing some turpitude, let him be whipped in public, deprived of his crown [tonsure] and, after having his head shaved, let his face be covered with spittle; and [let him be] bound in iron chains, condemned to six months in prison, reduced to eating rye bread once a day in the evening three times per week. After these six months living in a separate cell under the custody of a wise elder with great spiritual experience, let him be subjected to prayers, vigils and manual work, always under the guard of two spiritual brothers, without being allowed to have any relationship … with young people.” (St. Basil of Caesarea, in St. Peter Damien, Liber Gomorrhianus, op. cit. cols. 174f.)

Council of Toledo.

In the opening speech of the XVI Council of Toledo in 693, Egica, the Gothic King of Spain, exhorts the clergy to fight against homosexual practices: “See that you determine to extirpate that obscene crime committed by those who lie with males, whose fearful conduct defiles the charm of honest living and provokes from heaven the wrath of the Supreme Judge.”

Saint Peter Damian

 Liber Gomorrhianus [Book of Gomorrha], addressed to Pope Leo IX in the year 1051, is considered the principal work against homosexuality. It reads: “Just as Saint Basil establishes that those who incur sins [against nature] … should be subjected not only to a hard penance but a public one, and Pope Siricius prohibits penitents from entering clerical orders, one can clearly deduce that he who corrupts himself with a man through the ignominious squalor of a filthy union does not deserve to exercise ecclesiastical functions, since those who were formerly given to vices … become unfit to administer the Sacraments.” (St. Peter Damian, op. cit., cols. 174f) “This vice strives to destroy the walls of one’s heavenly motherland and rebuild those of devastated Sodom. Indeed, it violates temperance, kills purity, stifles chastity and annihilates virginity … with the sword of a most infamous union. It infects, stains and pollutes everything; it leaves nothing pure, there is nothing but filth … This vice expels one from the choir of the ecclesiastical host and obliges one to join the energumens and those who work in league with the devil; it separates the soul from God and links it with the demons. This most pestiferous queen of the Sodomites [which is homosexuality] makes those who obey her tyrannical laws repugnant to men and hateful to God … It humiliates at church, condemns at court, defiles in secret, dishonors in public, gnaws at the person’s conscience like a worm and burns his flesh like fire…  “The miserable flesh burns with the fire of lust, the cold intelligence trembles under the rancor of suspicion, and the unfortunate man’s heart is possessed by hellish chaos, and his pains of conscience are as great as the tortures in punishment he will suffer … Indeed, this scourge destroys the foundations of faith, weakens the force of hope, dissipates the bonds of charity, annihilates justice, undermines fortitude, … and dulls the edge of prudence.  “What else shall I say? It expels all the forces of virtue from the temple of the human heart and, pulling the door from its hinges, introduces into it all the barbarity of vice … In effect, the one whom … this atrocious beast [of homosexuality] has swallowed down its bloody throat is prevented, by the weight of his chains, from practicing all good works and is precipitated into the very abysses of its uttermost wickedness. Thus, as soon as someone has fallen into this chasm of extreme perdition, he is exiled from the heavenly motherland, separated from the Body of Christ, confounded by the authority of the whole Church, condemned by the judgment of all the Holy Fathers, despised by men on earth, and reproved by the society of heavenly citizens. He creates for himself an earth of iron and a sky of bronze … He cannot be happy while he lives nor have hope when he dies, because in life he is obliged to suffer the ignominy of men’s derision and later, the torment of eternal condemnation” (Liber Gomorrhianus, in PL 145, col. 159-178).

Saint Albert the Great

He gives four reasons why he considers homosexual acts as the most detestable ones: They are born from an ardent frenzy; they are disgustingly foul; those who become addicted to them are seldom freed from that vice; they are as contagious as disease, passing quickly from one person to another. (St. Albert the Great, In Evangelium Lucae XVII, 29, in J. McNeill, op. cit., p. 95)

St. Bernardine of Siena :

a preacher of the fifteenth century, makes a psychological analysis of the consequences of the homosexual vice. The illustrious Franciscan writes: “No sin has greater power over the soul than the one of cursed sodomy, which was always detested by all those who lived according to God….. Such passion for undue forms borders on madness. This vice disturbs the intellect, breaks an elevated and generous state of soul, drags great thoughts to petty ones, makes [men] pusillanimous and irascible, obstinate and hardened, servile soft and incapable of anything. Furthermore, the will, being agitated by the insatiable drive for pleasure, no longer follows reason, but furor…. Someone who lived practicing the vice of sodomy will suffer more pains in Hell than any one else, because this is the worst sin that there is.” (St. Bernardine of Siena, Predica XXXIX, in Le prediche volgari (Milan: Rizzoli, 1936), pp. 869ff., 915, in F. Bernadei, op. cit., pp. 11f)
 

Saint Thomas Aquinas,

Writing about sins against nature, explains: “However, they are called passions of ignominy because they are not worthy of being named, according to that passage in Ephesians (5:12): ‘For the things that are done by them in secret, it is a shame even to speak of.’ For if the sins of the flesh are commonly censurable because they lead man to that which is bestial in him, much more so is the sin against nature, by which man debases himself lower than even his animal nature.” (St. Thomas Aquinas, Super Epistulas Sancti Pauli Ad Romanum I, 26, pp. 27f)

Saint Bonaventure,

Speaking in a sermon at the church of Saint Mary of Portiuncula about the miracles that took place simultaneously with the birth of our Lord Jesus Christ, narrates this: “Seventh prodigy: All sodomites, men and women died all over the earth, as Saint Jerome said in his commentary on the psalm ‘The light was born for the just.’ This made it clear that He was born to reform nature and promote chastity.” (St. Bonaventure, Sermon XXIÑIn Nativitate Domini, in Catolicismo (Campos/Sao Paulo), December 1987, p. 3; F. Bernardei, op. cit., p. 11)

Saint Catherine of Siena,

A religious mystic of the 14th century, relays words of Our Lord Jesus Christ about the vice against nature, which contaminated part of the clergy in her time. Referring to sacred ministers, He says: “They not only fail from resisting this frailty [of fallen human nature] … but do even worse as they commit the cursed sin against nature. Like the blind and stupid, having dimmed the light of their understanding, they do not recognize the disease and misery in which they find themselves. For this not only causes me nausea, but displeases even the demons themselves, whom these miserable creatures have chosen as their lords. For me, this sin against nature is so abominable that, for it alone, five cities were submersed, by virtue of the judgment of My Divine Justice, which could no longer bear them…. It is disagreeable to the demons, not because evil displeases them and they find pleasure in good, but because their nature is angelic and thus is repulsed upon seeing such an enormous sin being committed. It is true that it is the demon who hits the sinner with the poisoned arrow of lust, but when a man carries out such a sinful act, the demon leaves.” (St. Catherine of Siena, El di‡logo, in Obras de Santa Catarina de Siena (Madrid: BAC, 1991), p. 292)

The Fifth Lateran Council [1512-1517] issued this decree:

 “Let any member of the clergy caught in that vice against nature, given that the wrath of God falls over the sons of perfidy, be removed from the clerical order or forced to do penance in a monastery” (chap. 4, X, V, 31).

Pope St. Pius V

Speaking of this sin, the great zealous defender of the rights of God states: "That horrible crime, by which corrupt and obscene cities were destroyed by fire through divine condemnation, [hence, “sodomy”] causes us most bitter sorrow and shocks our mind, impelling us to repress such a crime with the greatest possible zeal." - On August 30, 1568, in his Papal the bull Horrendum Illud scelus.

The Code of Canon Law

Undertaken at the initiative and encouragement of Saint Pius X, and published in 1917 by his successor Pope Benedict XV, says this: “So far as laymen are concerned, the sin of sodomy is punished ipso facto with the pain of infamy and other sanctions to be applied according to the prudent judgment of the Bishop depending on the gravity of each case (Can. 2357). As for ecclesiastics and religious, if they are clerici minoris [that is, of a degree lower than deacon], let them be punished with various measures, proportional to the gravity of the fault, that can even include dismissal from the clerical state (Can. 2358); if they are clerici maiores [that is, deacons, priests or bishops], let them ‘be declared infamous and suspended from every post, benefit, dignity, deprived of their eventual stipend and, in the gravest cases, let them be deposed’ (Can. 2359, par. 2).”

(1)  De adult. conjug.; cap. Adulterii; cause 32; q 7; quoted by St. Thomas Aquinas: Summa Theologica; II II q 154, a 12.

(2)  St. Thomas Aquinas: De Malo; q xv, a 2, ad 6.

(3)  Pope Alexander VII: Decree from the Holy Office, Sept. 24, 1665 prop. No. 24. (Dzs. 2044).