Objection 1. It would seem that the marriage act cannot be altogether excused from sin by the aforesaid goods. For whoever allows himself to lose a greater good for the sake of a lesser good sins because he allows it inordinately. Now the good of reason which is prejudiced in the marriage act is greater than these three marriage goods. Therefore the aforesaid goods do not suffice to excuse marriage intercourse.
Objection 2. Further, if a moral good be added to a moral evil the sum total is evil and not good, since one evil circumstance makes an action evil, whereas one good circumstance does not make it good. Now the marriage act is evil in itself, else it would need no excuse. Therefore the addition of the marriage goods cannot make the act good.
Objection 3. Further, wherever there is immoderate passion there is moral vice. Now the marriage goods cannot prevent the pleasure in that act from being immoderate. Therefore they cannot excuse it from being a sin.
Objection 4. Further, according to Damascene (De Fide Orth. ii, 15), shame is only caused by a disgraceful deed. Now the marriage goods do not deprive that deed of its shame. Therefore they cannot excuse it from sin.
On the contrary, The marriage act differs not from fornication except by the marriage goods. If therefore these were not sufficient to excuse it marriage would be always unlawful; and this is contrary to what was stated above (Question 41, Article 3).
Further, the marriage goods are related to its act as its due circumstances, as stated above (1, ad 4). Now the like circumstances are sufficient to prevent an action from being evil. Therefore these goods can excuse marriage so that it is nowise a sin.
I answer that, An act is said to be excused in two ways. First, on the part of the agent, so that although it be evil it is not imputed as sin to the agent, or at least not as so grave a sin. thus ignorance is said to excuse a sin wholly or partly. Secondly, an act is said to be excused on its part, so that, namely, it is not evil; and it is thus that the aforesaid goods are said to excuse the marriage act. Now it is from the same cause that an act is not morally evil, and that it is good, since there is no such thing as an indifferent act, as was stated in the Second Book (Sent. ii, D, 40; I-II, 18, 9). Now a human act is said to be good in two ways. In one way by goodness of virtue, and thus an act derives its goodness from those things which place it in the mean. This is what "faith" and "offspring" do in the marriage act, as stated above (Article 2). In another way, by goodness of the "sacrament," in which way an act is said to be not only good, but also holy, and the marriage act derives this goodness from the indissolubility of the union, in respect of which it signifies the union of Christ with the Church. Thus it is clear that the aforesaid goods sufficiently excuse the marriage act.
Reply to Objection 1. By the marriage act man does not incur harm to his reason as to habit, but only as to act. Nor is it unfitting that a certain act which is generically better be sometimes interrupted for some less good act; for it is possible to do this without sin, as in the case of one who ceases from the act of contemplation in order meanwhile to devote himself to action.
Reply to Objection 2. This argument would avail if the evil that is inseparable from carnal intercourse were an evil of sin. But in this case it is an evil not of sin but of punishment alone, consisting in the rebellion of concupiscence against reason; and consequently the conclusion does not follow.
Reply to Objection 3. The excess of passion that amounts to a sin does not refer to the passion's quantitative intensity, but to its proportion to reason; wherefore it is only when a passion goes beyond the bounds of reason that it is reckoned to be immoderate. Now the pleasure attaching to the marriage act, while it is most intense in point of quantity, does not go beyond the bounds previously appointed by reason before the commencement of the act, although reason is unable to regulate them during the pleasure itself.
Reply to Objection 4. The turpitude that always accompanies the marriage act and always causes shame is the turpitude of punishment, not of sin, for man is naturally ashamed of any defect.
Source: Summa Theologica (New Advent)