2 The Lord, in that He came to the marriage to which He was invited, wished, apart from the mystical signification, to assure us that marriage was His own institution. For there were to be those of whom the apostle spoke, “forbidding to marry,” and asserting that marriage was an evil, and of the devil's institution: notwithstanding the same Lord declares in the Gospel, on being asked whether it be lawful for a man to put away his wife for any cause, that it is not lawful save for the cause of fornication.
In His answer, if you remember, He said, “What God has joined together let not man put asunder.” And they that are well instructed in the catholic faith know that God instituted marriage; and as the union of man and wife is from God, so divorce is from the devil. But in the case of fornication it is lawful for a man to put away his wife, because she first chose to be no longer wife in not preserving conjugal fidelity to her husband. Nor are those women who vow virginity to God, although they hold a higher place of honor and sanctity in the Church, without marriage.
For they too, together with the whole Church, attain to a marriage, a marriage in which Christ is the Bridegroom. And for this cause, therefore, did the Lord, on being invited, come to the marriage, to confirm conjugal chastity, and to show forth the sacrament of marriage. For the bridegroom in that marriage, to whom it was said, “You have kept the good wine until now,” represented the person of the Lord. For the good wine— namely, the gospel— Christ has kept until now.
Source: Tractates on the Gospel of John (New Advent)