The “sin of the Sodomites” isn’t specified in the CCC and it’s wasn’t necessarily what we call sodomy anyway (unless you ignore Ezekiel).
Sodomy has always been understood as homosexuality. It was the worse offense of the people of Sodom mentioned in the book of Ezekiel.
St. Augustine:
“Those offences which be contrary to nature are everywhere and at all times to be held in detestation and punished; such were those of the Sodomites, which should all nations commit, they should all be held guilty of the same crime by the divine law, which hath not so made men that they should in that way abuse one another. For even that fellowship which should be between God and us is violated, when that same nature of which He is author is polluted by the perversity of lust.”
St. John Chrysostom:
“Consider how great is that sin, to have forced hell to appear even before its time!… For that rain was unwonted, for the intercourse was contrary to nature, and it deluged the land, since lust had done so with their souls. Wherefore also the rain was the opposite of the customary rain. Now not only did it fail to stir up the womb of the earth to the production of fruits, but made it even useless for the reception of seed. For such was also the intercourse of the men, making a body of this sort more worthless than the very land of Sodom. And what is there more detestable than a man who hath pandered himself, or what more execrable?”
St. Gregory the Great:
“Sacred Scripture itself confirms that sulfur evokes the stench of the flesh, as it speaks of the rain of fire and sulfur poured upon Sodom by the Lord. He had decided to punish Sodom for the crimes of the flesh, and the very type of punishment he chose emphasized the shame of that crime. For sulfur stinks, and fire burns. So it was just that Sodomites, burning with perverse desires arising from the flesh like stench, should perish by fire and sulfur so that through this just punishment they would realize the evil they had committed, led by a perverse desire.”
St. Catherine of Sienna,quoting Jesus’ words to her:
“But they act in a contrary way, for they come full of impurity to this mystery, and not only of that impurity to which, through the fragility of your weak nature, you are all naturally inclined (although reason, when free will permits, can quiet the rebellion of nature), but these wretches not only do not bridle this fragility, but do worse, committing that accursed sin against nature, and as blind and fools, with the light of their intellect darkened, they do not know the stench and misery in which they are. It is not only that this sin stinks before me, who am the Supreme and Eternal Truth, it does indeed displease me so much and I hold it in such abomination that for it alone I buried five cities by a divine judgment, my divine justice being no longer able to endure it. This sin not only displeases me as I have said, but also the devils whom these wretches have made their masters. Not that the evil displeases them because they like anything good, but because their nature was originally angelic, and their angelic nature causes them to loathe the sight of the actual commission of this enormous sin.”
St. Peter Canisius:
“No sin in the world grips the soul as the accursed sodomy; this sin has always been detested by all those who live according to God.… Deviant passion is close to madness; this vice disturbs the intellect, destroys elevation and generosity of soul, brings the mind down from great thoughts to the lowliest, makes the person slothful, irascible, obstinate and obdurate, servile and soft and incapable of anything; furthermore, agitated by an insatiable craving for pleasure, the person follows not reason but frenzy.… They become blind and, when their thoughts should soar to high and great things, they are broken down and reduced to vile and useless and putrid things, which could never make them happy… Just as people participate in the glory of God in different degrees, so also in hell some suffer more than others. He who lived with this vice of sodomy suffers more than another, for this is the greatest sin.”
That said, sodomy as we usually define it (oral and anal sex) is allowable by Catholic teaching as long as the man “completes” in the proper place.
Where did you get that idea? That is not a Catholic teaching. Even married couples must act within the bounds of natural law morality and purity and modesty.
St. Thomes Aquinas:
"If all the sins of the flesh are worthy of condemnation because by them man allows himself to be dominated by that which he has of the animal nature, much more deserving of condemnation are the sins against nature by which man degrades his own animal nature…
"Man can sin against nature in two ways. First, when he sins against his specific rational nature, acting contrary to reason. In this sense, we can say that every sin is a sin against man’s nature, because it is against man’s right reason…
“Secondly, man sins against nature when he goes against his generic nature, that is to say, his animal nature. Now, it is evident that, in accord with natural order, the union of the sexes among animals is ordered towards conception. From this it follows that every sexual intercourse that cannot lead to conception is opposed to man’s animal nature.”