Catholic fine art - The Marriage of Mary and Joseph*
Third Part of St. Thomas Aquinas Summa Theologica:
Marriage or wedlock is said to be true by reason of its attaining its perfection. Now perfection of anything is twofold; first, and second. The first perfection of a thing consists in its very form, from which it receives its species; while the second perfection of a thing consists in its operation, by which in some way a thing attains its end. Now the form of matrimony consists in a certain inseparable union of souls, by which husband and wife are pledged by a bond of mutual affection that cannot be sundered. And the end of matrimony is the begetting and upbringing of children: the first of which is attained by conjugal intercourse; the second by the other duties of husband and wife, by which they help one another in rearing their offspring.
Thus we may say, as to the first perfection, that the marriage of the Virgin Mother of God and Joseph was absolutely true: because both consented to the nuptial bond, but not expressly to the bond of the flesh, save on the condition that it was pleasing to God. For this reason the angel calls Mary the wife of Joseph, saying to him (Mat. 1:20): "Fear not to take unto thee Mary thy wife": on which words Augustine says (De Nup. et Concup. i): "She is called his wife from the first promise of her espousals, whom he had not known nor ever was to know by carnal intercourse."
But as to the second perfection which is attained by the marriage act, if this be referred to carnal intercourse, by which children are begotten; thus this marriage was not consummated. Wherefore Ambrose says on Lk. 1:26,27: "Be not surprised that Scripture calls Mary a wife. The fact of her marriage is declared, not to insinuate the loss of virginity, but to witness to the reality of the union." Nevertheless, this marriage had the second perfection, as to upbringing of the child. Thus Augustine says (De Nup. et Concup. i): "All the nuptial blessings are fulfilled in the marriage of Christ's parents, offspring, faith and sacrament. The offspring we know to have been the Lord Jesus; faith, for there was no adultery: sacrament, since there was no divorce. Carnal intercourse alone there was none."