...every act of sexual lust is a sin either because of the disorder of the act or even because of the disorder of teh desire alone, which disorder primarily and intrinsically belongs to sexual lust. For Augustine says in the City of God: "Sexual lust is not the sin of beautiful and pleasant bodies but of souls wickedly loving bodily pleasures to the neglect of moderation, which makes us fit for things that are spiritually more beautiful and pleasant."
Thomas Aquinas, De Malo 15.1.
[Thomas Aquinas, On Evil, Regan, tr., Oxford University Press (Oxford: 2003) p. 422.]