Objection 1. It would seem that the marriage act is not meritorious. For Chrysostom [Hom. i in the Opus Imperfectum, falsely ascribed to St. John Chrysostom] says in his commentary on Matthew: "Although marriage brings no punishment to those who use it, it affords them no meed." Now merit bears a relation to meed. Therefore the marriage act is not meritorious.
Objection 2. Further, to refrain from what is meritorious deserves not praise. Yet virginity whereby one refrains from marriage is praiseworthy. Therefore the marriage act is not meritorious.
Objection 3. Further, he who avails himself of an indulgence granted him, avails himself of a favor received. But a man does not merit by receiving a favor. Therefore the marriage act is not meritorious.
Objection 4. Further, merit like virtue, consists in difficulty. But the marriage act affords not difficulty but pleasure. Therefore it is not meritorious.
Objection 5. Further, that which cannot be done without venial sin is never meritorious, for a man cannot both merit and demerit at the same time. Now there is always a venial sin in the marriage act, since even the first movement in such like pleasures is a venial sin. Therefore the aforesaid act cannot be meritorious.
On the contrary, Every act whereby a precept is fulfilled is meritorious if it be done from charity. Now such is the marriage act, for it is said (1 Corinthians 7:3): "Let the husband render the debt to his wife." Therefore, etc.
Further, every act of virtue is meritorious. Now the aforesaid act is an act of justice, for it is called the rendering of a debt. Therefore it is meritorious.
I answer that, Since no act proceeding from a deliberate will is indifferent, as stated in the Second Book (Sent. ii, D, 40, 1, 3; I-II, 18, 9), the marriage act is always either sinful or meritorious in one who is in a state of grace. For if the motive for the marriage act be a virtue, whether of justice that they may render the debt, or of religion, that they may beget children for the worship of God, it is meritorious. But if the motive be lust, yet not excluding the marriage blessings, namely that he would by no means be willing to go to another woman, it is a venial sin; while if he exclude the marriage blessings, so as to be disposed to act in like manner with any woman, it is a mortal sin. And nature cannot move without being either directed by reason, and thus it will be an act of virtue, or not so directed, and then it will be an act of lust.
Reply to Objection 1. The root of merit, as regards the essential reward, is charity itself; but as regards an accidental reward, the reason for merit consists in the difficulty of an act; and thus the marriage act is not meritorious except in the first way.
Reply to Objection 2. The difficulty required for merit of the accidental reward is a difficulty of labor, but the difficulty required for the essential reward is the difficulty of observing the mean, and this is the difficulty in the marriage act.
Reply to Objection 3. First movements in so far as they are venial sins are movements of the appetite to some inordinate object of pleasure. This is not the case in the marriage act, and consequently the argument does not prove.
Source: Summa Theologica (New Advent)