First of all let me say, like St. Faustina in Divine Mercy Diary, “the greater the sinner, the greater his right to God’s mercy.” “There is no sin too great for God’s mercy.”
So don’t despair for anyone committing Sodomy or any other grave sin, all can be overcome with the love of God.
In regards to your specific question, all coming from St. Thomas in the Summa, under his listing of the
order of gravity of sins.
newadvent.org/summa/3154.htm#article12
that “of all these,” namely the sins belonging to lust, “that which is against nature is the worst.”
(FYI) The saint goes on to say what is against nature… and sodomy is one.
Hence Augustine says (Confess. iii, 8): “Those foul offenses that are against nature should be everywhere and at all times detested and punished, such as were those of the people of
Sodom, which should all nations commit, they should all stand guilty of the same crime, by the law of God which hath not so made men that they should so abuse one another. For even that very intercourse which should be between God and us is violated, when that same nature, of which He is the Author, is polluted by the perversity of lust.”
St. Thomas Aquinas on speaking of lust says,
Gravity of a sin depends more on the abuse of a thing than on the omission of the right use. Wherefore among sins against nature, the lowest place belongs to the sin of uncleanness, which consists in the mere omission of copulation with another. While the most grievous is the sin of bestiality, because use of the due species is not observed. Hence a gloss on Genesis 37:2, “He accused his brethren of a most wicked crime,” says that “they copulated with cattle.” After this comes the sin of
sodomy, because use of the right sex is not observed. Lastly comes the sin of not observing the right manner of copulation, which is more grievous if the abuse regards the “vas” than if it affects the manner of copulation in respect of other circumstances.
