What was St. Thomas Aquinas' view on homosexuality?
Referring to at as "the unnatural vice" he calls it the greatest sin among the sins of lust.
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: [...] secondly, because 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[...] 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.
Since by the unnatural vices man transgresses that which has been determined by nature with regard to the use of venereal actions, it follows that in this matter this sin is gravest of all. After it comes incest, which, as stated above (Article 9), is contrary to the natural respect which we owe persons related to us.