Objection 1. It would seem that Matrimony ought not to have been instituted before sin. Because that which is of natural law needs not to be instituted. Now such is Matrimony, as stated above (Question 41, Article 1). Therefore it ought not to have been instituted.
Objection 2. Further, sacraments are medicines against the disease of sin. But a medicine is not made ready except for an actual disease. Therefore it should not have been instituted before sin.
Objection 3. Further, one institution suffices for one thing. Now Matrimony was instituted also after sin, as stated in the text (Sent. iv, D, 26). Therefore it was not instituted before sin.
Objection 4. Further, the institution of a sacrament must come from God. Now before sin, the words relating to Matrimony were not definitely said by God but by Adam; the words which God uttered (Genesis 1:22), "Increase and multiply," were addressed also to the brute creation where there is no marriage. Therefore Matrimony was not instituted before sin.
Objection 5. Further, Matrimony is a sacrament of the New Law. But the sacraments of the New Law took their origin from Christ. Therefore it ought not to have been instituted before sin.
On the contrary, It is said (Matthew 19:4): "Have ye not read that He Who made man from the beginning 'made them male and female'"?
Further, Matrimony was instituted for the begetting of children. But the begetting of children was necessary to man before sin. Therefore it behooved Matrimony to be instituted before sin.
I answer that, Nature inclines to marriage with a certain good in view, which good varies according to the different states of man, wherefore it was necessary for matrimony to be variously instituted in the various states of man in reference to that good. Consequently matrimony as directed to the begetting of children, which was necessary even when there was no sin, was instituted before sin; according as it affords a remedy for the wound of sin, it was instituted after sin at the time of the natural law; its institution belongs to the Mosaic Law as regards personal disqualifications; and it was instituted in the New Law in so far as it represents the mystery of Christ's union with the Church, and in this respect it is a sacrament of the New Law. As regards other advantages resulting from matrimony, such as the friendship and mutual services which husband and wife render one another, its institution belongs to the civil law. Since, however, a sacrament is essentially a sign and a remedy, it follows that the nature of sacrament applies to matrimony as regards the intermediate institution; that it is fittingly intended to fulfill an office of nature as regards the first institution; and. as regards the last-mentioned institution, that it is directed to fulfill an office of society.
Reply to Objection 1. Things which are of natural law in a general way, need to be instituted as regards their determination which is subject to variation according to various states; just as it is of natural law that evil-doers be punished, but that such and such a punishment be appointed for such and such a crime is determined by positive law.
Reply to Objection 2. Matrimony is not only for a remedy against sin, but is chiefly for an office of nature; and thus it was instituted before sin, not as intended for a remedy.
Reply to Objection 3. There is no reason why matrimony should not have had several institutions corresponding to the various things that had to be determined in connection with marriage. Hence these various institutions are not of the same thing in the same respect.
Reply to Objection 4. Before sin matrimony was instituted by God, when He fashioned a helpmate for man out of his rib, and said to them: "Increase and multiply." And although this was said also to the other animals, it was not to be fulfilled by them in the same way as by men. As to Adam's words, he uttered them inspired by God to understand that the institution of marriage was from God.
Reply to Objection 5. As was clearly stated, matrimony was not instituted before Christ as a sacrament of the New Law.
Source: Summa Theologica (New Advent)