WIST

Natural inclinations are present in things from God, who moves all things. So it is impossible for the natural inclinations of a species to be toward evil in itself. But there is in all perfect animals a natural inclination toward carnal union. Therefore it is impossible for carnal union to be evil in itself.

Thomas Aquinas (1225-1274) Italian friar, philosopher, theologian
Summa Contra Gentiles, Book 3, ch. 126, argument 3 [tr. Dominican (1923)]
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Alt. trans.: "Natural inclinations are put into things by God, who is the prime mover of all. Therefore it is impossible for the natural inclination of any species to be directed to an object in itself evil. But in all full-grown animals there is a natural inclination to sexual union, which union therefore cannot be in itself evil."