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.
On the contrary, The cause is more powerful than its effect. Now wine is forbidden on account of lust, according to the saying of the Apostle (Ephesians 5:18), "Be not drunk with wine wherein is lust [Douay: 'luxury']." Therefore lust is forbidden.
Further, it is numbered among the works of the flesh: Galatians 5:19 [Douay: 'luxury'].
I answer that, The more necessary a thing is, the more it behooves one to observe the order of reason in its regard; wherefore the more sinful it becomes if the order of reason be forsaken. Now the use of venereal acts, as stated in the foregoing Article, is most necessary for the common good, namely the preservation of the human race. Wherefore there is the greatest necessity for observing the order of reason in this matter: so that if anything be done in this connection against the dictate of reason's ordering, it will be a sin. Now lust consists essentially in exceeding the order and mode of reason in the matter of venereal acts. Wherefore without any doubt lust is a sin.
Reply to Objection 1. As the Philosopher says in the same book (De Gener. Anim. i, 18), "the semen is a surplus that is needed." For it is said to be superfluous, because it is the residue from the action of the nutritive power, yet it is needed for the work of the generative power. But the other superfluities of the human body are such as not to be needed, so that it matters not how they are emitted, provided one observe the decencies of social life. It is different with the emission of semen, which should be accomplished in a manner befitting the end for which it is needed.
Reply to Objection 2. As the Apostle says (1 Corinthians 6:20) in speaking against lust, "You are bought with a great price: glorify and bear God in your body." Wherefore by inordinately using the body through lust a man wrongs God Who is the Supreme Lord of our body. Hence Augustine says (De Decem. Chord. 10 [Serm. ix (xcvi de Temp.)]): "God Who thus governs His servants for their good, not for His, made this order and commandment, lest unlawful pleasures should destroy His temple which thou hast begun to be."
Reply to Objection 3. The opposite of lust is not found in many, since men are more inclined to pleasure. Yet the contrary vice is comprised under insensibility, and occurs in one who has such a dislike for sexual intercourse as not to pay the marriage debt.
Source: Summa Theologica (New Advent)