Objection 1. It would seem that certain blessings are not necessary in order to excuse marriage. For just as the preservation of the individual which is effected by the nutritive power is intended by nature, so too is the preservation of the species which is effected by marriage; and indeed so much the more as the good of the species is better and more exalted than the good of the individual. But no goods are necessary to excuse the act of the nutritive power. Neither therefore are they necessary to excuse marriage.
Objection 2. Further, according to the Philosopher (Ethic. viii, 12) the friendship between husband and wife is natural, and includes the virtuous, the useful, and the pleasant. But that which is virtuous in itself needs no excuse. Therefore neither should any goods be assigned for the excuse of matrimony.
Objection 3. Further, matrimony was instituted as a remedy and as an office, as stated above (Question 42, Article 2). Now it needs no excuse in so far as it is instituted as an office, since then it would also have needed an excuse in paradise, which is false, for there, as Augustine says, "marriage would have been without reproach and the marriage-bed without stain" (Gen. ad lit. ix). In like manner neither does it need an excuse in so far as it is intended as a remedy, any more than the other sacraments which were instituted as remedies for sin. Therefore matrimony does not need these excuses.
Objection 4. Further, the virtues are directed to whatever can be done aright. If then marriage can be righted by certain goods, it needs nothing else to right it besides the virtues of the soul; and consequently there is no need to assign to matrimony any goods whereby it is righted, any more than to other things in which the virtues direct us.
On the contrary, Wherever there is indulgence, there must needs be some reason for excuse. Now marriage is allowed in the state of infirmity "by indulgence" (1 Corinthians 7:6). Therefore it needs to be excused by certain goods.
Further, the intercourse of fornication and that of marriage are of the same species as regards the species of nature. But the intercourse of fornication is wrong in itself. Therefore, in order that the marriage intercourse be not wrong, something must be added to it to make it right, and draw it to another moral species.
I answer that, No wise man should allow himself to lose a thing except for some compensation in the shape of an equal or better good. Wherefore for a thing that has a loss attached to it to be eligible, it needs to have some good connected with it, which by compensating for that loss makes that thing ordinate and right. Now there is a loss of reason incidental to the union of man and woman, both because the reason is carried away entirely on account of the vehemence of the pleasure, so that it is unable to understand anything at the same time, as the Philosopher says (Ethic. vii, 11); and again because of the tribulation of the flesh which such persons have to suffer from solicitude for temporal things (1 Corinthians 7:28). Consequently the choice of this union cannot be made ordinate except by certain compensations whereby that same union is righted. and these are the goods which excuse marriage and make it right.
Reply to Objection 1. In the act of eating there is not such an intense pleasure overpowering the reason as in the aforesaid action, both because the generative power, whereby original sin is transmitted, is infected and corrupt, whereas the nutritive power, by which original sin is not transmitted, is neither corrupt nor infected; and again because each one feels in himself a defect of the individual more than a defect of the species. Hence, in order to entice a man to take food which supplies a defect of the individual, it is enough that he feel this defect; but in order to entice him to the act whereby a defect of the species is remedied, Divine providence attached pleasure to that act, which moves even irrational animals in which there is not the stain of original sin. Hence the comparison fails.
Reply to Objection 2. These goods which justify marriage belong to the nature of marriage, which consequently needs them, not as extrinsic causes of its rectitude, but as causing in it that rectitude which belongs to it by nature.
Reply to Objection 3. From the very fact that marriage is intended as an office or as a remedy it has the aspect of something useful and right; nevertheless both aspects belong to it from the fact that it has these goods by which it fulfills the office and affords a remedy to concupiscence.
Reply to Objection 4. An act of virtue may derive its rectitude both from the virtue as its elicitive principle, and from its circumstances as its formal principles; and the goods of marriage are related to marriage as circumstances to an act of virtue which owes it to those circumstances that it can be an act of virtue.
Source: Summa Theologica (New Advent)