Whether the lust that is about venereal acts can be a sin?

❌ Objection 1 : It would seem that lust about venereal acts cannot be a sin. For the venereal act consists in the emission of semen which is the surplus from food, according to the Philosopher (De Gener. Anim. i, 18). But there is no sin attaching to the emission of other superfluities. Therefore neither can there be any sin in venereal acts.

❌ Objection 2 : Further, everyone can lawfully make what use he pleases of what is his. But in the venereal act a man uses only what is his own, except perhaps in adultery or rape. Therefore there can be no sin in venereal acts, and consequently lust is no sin.

❌ Objection 3 : Further, every sin has an opposite vice. But, seemingly, no vice is opposed to lust. Therefore lust is not a sin.