Objection 1. It would seem that the marriage act is always sinful. For it is written (1 Corinthians 7:29): "That they... who have wives, be as if they had none." But those who are not married do not perform the marriage act. Therefore even those who are married sin in that act.
Objection 2. Further, "Your iniquities have divided between you and your God." Now the marriage act divides man from God wherefore the people who were to see God (Exodus 19:11) were commanded not to go near their wives (Exodus 19:20); and Jerome says (Ep. ad Ageruch.: Contra Jovini, 18) that in the marriage act "the Holy Ghost touches not the hearts of the prophets." Therefore it is sinful.
Objection 3. Further, that which is shameful in itself can by no means be well done. Now the marriage act is always connected with concupiscence, which is always shameful. Therefore it is always sinful.
Objection 4. Further, nothing is the object of excuse save sin. Now the marriage act needs to be excused by the marriage blessings, as the Master says (Sent. iv, D, 26). Therefore it is a sin.
Objection 5. Further, things alike in species are judged alike. But marriage intercourse is of the same species as the act of adultery, since its end is the same, namely the human species. Therefore since the act of adultery is a sin, the marriage act is likewise.
Objection 6. Further, excess in the passions corrupts virtue. Now there is always excess of pleasure in the marriage act, so much so that it absorbs the reason which is man's principal good, wherefore the Philosopher says (Ethic. vii, 11) that "in that act it is impossible to understand anything." Therefore the marriage act is always a sin.
On the contrary, It is written (1 Corinthians 7:28): "If a virgin marry she hath not sinned," and (1 Timothy 5:14): "I will... that the younger should marry," and "bear children." But there can be no bearing of children without carnal union. Therefore the marriage act is not a sin; else the Apostle would not have approved of it.
Further, no sin is a matter of precept. But the marriage act is a matter of precept (1 Corinthians 7:3): "Let the husband render the debt to his life." Therefore it is not a sin.
I answer that, If we suppose the corporeal nature to be created by the good God we cannot hold that those things which pertain to the preservation of the corporeal nature and to which nature inclines, are altogether evil; wherefore, since the inclination to beget an offspring whereby the specific nature is preserved is from nature, it is impossible to maintain that the act of begetting children is altogether unlawful, so that it be impossible to find the mean of virtue therein; unless we suppose, as some are mad enough to assert, that corruptible things were created by an evil god, whence perhaps the opinion mentioned in the text is derived (Sent. iv, D, 26); wherefore this is a most wicked heresy.
Reply to Objection 1. By these words the Apostle did not forbid the marriage act, as neither did he forbid the possession of things when he said (1 Corinthians 7:31): "They that use this world" (let them be) "as if they used it not." In each case he forbade enjoyment ["Fruitionem," i.e. enjoyment of a thing sought as one's last end]; which is clear from the way in which he expresses himself; for he did not say "let them not use it," or "let them not have them," but let them be "as if they used it not" and "as if they had none."
Reply to Objection 2. We are united to God by the habit of grace and by the act of contemplation and love. Therefore whatever severs the former of these unions is always a sin, but not always that which severs the latter, since a lawful occupation about lower things distracts the mind so that it is not fit for actual union with God; and this is especially the case in carnal intercourse wherein the mind is withheld by the intensity of pleasure. For this reason those who have to contemplate Divine things or handle sacred things are enjoined not to have to do with their wives for that particular time; and it is in this sense that the Holy Ghost, as regards the actual revelation of hidden things, did not touch the hearts of the prophets at the time of the marriage act.
Reply to Objection 3. The shamefulness of concupiscence that always accompanies the marriage act is a shamefulness not of guilt, but of punishment inflicted for the first sin, inasmuch as the lower powers and the members do not obey reason. Hence the argument does not prove.
Reply to Objection 4. Properly speaking, a thing is said to be excused when it has some appearance of evil, and yet is not evil, or not as evil as it seems, because some things excuse wholly, others in part. And since the marriage act, by reason of the corruption of concupiscence, has the appearance of an inordinate act, it is wholly excused by the marriage blessing, so as not to be a sin.
Reply to Objection 5. Although they are the same as to their natural species, they differ as to their moral species, which differs in respect of one circumstance, namely intercourse with one's wife and with another than one's wife; just as to kill a man by assault or by justice differentiates the moral species, although the natural species is the same; and yet the one is lawful and the other unlawful.
Reply to Objection 6. The excess of passions that corrupts virtue not only hinders the act of reason, but also destroys the order of reason. The intensity of pleasure in the marriage act does not do this, since, although for the moment man is not being directed, he was previously directed by his reason.
Source: Summa Theologica (New Advent)