Objection 1. It would seem that matrimony does not confer grace. For, according to Hugh (De Sacram. i) "the sacraments, by virtue of their sanctification, confer an invisible grace." But matrimony has no sanctification essential to it. Therefore grace is not conferred therein.
Objection 2. Further, every sacrament that confers grace confers it by virtue of its matter and form. Now the acts which are the matter in this sacrament are not the cause of grace (for it would be the heresy of Pelagius to assert that our acts cause grace); and the words expressive of consent are not the cause of grace, since no sanctification results from them. Therefore grace is by no means given in matrimony.
Objection 3. Further, the grace that is directed against the wound of sin is necessary to all who have that wound. Now the wound of concupiscence is to be found in all. Therefore if grace were given in matrimony against the wound of concupiscence, all men ought to contract marriage, and it would be very stupid to refrain from matrimony.
Objection 4. Further, sickness does not seek a remedy where it finds aggravation. Now concupiscence is aggravated by concupiscence, because, according to the Philosopher (Ethic. iii, 12), "the desire of concupiscence is insatiable, and is increased by congenial actions." Therefore it would seem that grace is not conferred in matrimony, as a remedy for concupiscence.
On the contrary, Definition and thing defined should be convertible. Now causality of grace is included in the definition of a sacrament. Since, then, matrimony is a sacrament, it is a cause of grace.
Further, Augustine says (De Bono Viduit. viii; Gen. ad lit. ix, 7) that "matrimony affords a remedy to the sick." But it is not a remedy except in so far as it has some efficacy. Therefore it has some efficacy for the repression of concupiscence. Now concupiscence is not repressed except by grace. Therefore grace is conferred therein.
I answer that, There have been three opinions on this point. For some [Peter Lombard, Sent. iv, D, 2 said that matrimony is nowise the cause of grace, but only a sign thereof. But this cannot be maintained, for in that case it would in no respect surpass the sacraments of the Old Law. Wherefore there would be no reason for reckoning it among the sacraments of the New Law; since even in the Old Law by the very nature of the act it was able to afford a remedy to concupiscence lest the latter run riot when held in too strict restraint.
Hence others [St. Albert Magnus, Sent. iv, D, 26 said that grace is conferred therein as regards the withdrawal from evil, because the act is excused from sin, for it would be a sin apart from matrimony. But this would be too little, since it had this also in the Old Law. And so they say that it makes man withdraw from evil, by restraining the concupiscence lest it tend to something outside the marriage blessings, but that this grace does not enable a man to do good works. But this cannot be maintained, since the same grace hinders sin and inclines to good, just as the same heat expels cold and gives heat.
Hence others [St. Bonaventure, Sent. iv, D, 26 say that matrimony, inasmuch as it is contracted in the faith of Christ, is able to confer the grace which enables us to do those works which are required in matrimony. and this is more probable, since wherever God gives the faculty to do a thing, He gives also the helps whereby man is enabled to make becoming use of that faculty; thus it is clear that to all the soul's powers there correspond bodily members by which they can proceed to act. Therefore, since in matrimony man receives by Divine institution the faculty to use his wife for the begetting of children, he also receives the grace without which he cannot becomingly do so; just as we have said of the sacrament of orders (35, 1). And thus this grace which is given is the last thing contained in this sacrament.
Reply to Objection 1. Just as the baptismal water by virtue of its contact with Christ's body [Cf. III, 66, 3, ad 4] is able to "touch the body and cleanse the heart" [St. Augustine, Tract. lxxx in Joan.], so is matrimony able to do so through Christ having represented it by His Passion, and not principally through any blessing of the priest.
Reply to Objection 2. Just as the water of Baptism together with the form of words results immediately not in the infusion of grace, but in the imprinting of the character, so the outward acts and the words expressive of consent directly effect a certain tie which is the sacrament of matrimony; and this tie by virtue of its Divine institution works dispositively [Cf. 18, 1], where St. Thomas uses the same expression; and Editor's notes at the beginning of the Supplement and on that Article] to the infusion of grace.
Reply to Objection 3. This argument would hold if no more efficacious remedy could be employed against the disease of concupiscence; but a yet more powerful remedy is found in spiritual works and mortification of the flesh by those who make no use of matrimony.
Reply to Objection 4. A remedy can be employed against concupiscence in two ways. First, on the part of concupiscence by repressing it in its root, and thus matrimony affords a remedy by the grace given therein. Secondly, on the part of its act, and this in two ways: first, by depriving the act to which concupiscence inclines of its outward shamefulness, and this is done by the marriage blessings which justify carnal concupiscence; secondly, by hindering the shameful act, which is done by the very nature of the act. because concupiscence, being satisfied by the conjugal act, does not incline so much to other wickedness. For this reason the Apostle says (1 Corinthians 7:9): "It is better to marry than to burn." For though the works congenial to concupiscence are in themselves of a nature to increase concupiscence, yet in so far as they are directed according to reason they repress concupiscence, because like acts result in like dispositions and habits.
Source: Summa Theologica (New Advent)