Everytime I ask this, people say “It’s in the Bible”, without providing context or explanation, and start (mis)quoting verses.
What is wrong with two men/women loving each other? How can God regard love as an (as some Christians say of homosexuals) “abomination”?
St. Thomas Aquinas, Summa Theologica, II, II,
Question 154. The parts of Lust
Article 11. Whether the unnatural vice is a species of lust?
I answer that, As stated above (Articles 6 and 9) wherever there occurs a special kind of deformity whereby the venereal act is rendered unbecoming, there is a determinate species of lust. This may occur in two ways: * First, through being contrary to right reason, and this is common to all lustful vices; * secondly, because, in addition, it is contrary to the natural order of the venereal act as becoming to the human race: and this is called “the unnatural vice.” This may happen in several ways.* First, by procuring pollution, without any copulation, for the sake of venereal pleasure: this pertains to the sin of “uncleanness” which some call “effeminacy”* Secondly, by copulation with a thing of undue species, and this is called “bestiality.” * Thirdly, by copulation with an undue sex, male with male, or female with female, as the Apostle states (Romans 1:27): and this is called the “vice of sodomy.” * Fourthly, by not observing the natural manner of copulation, either as to undue means, or as to other monstrous and bestial manners of copulation.
newadvent.org/summa/3154.htm#article11