Objection 1. It would seem that there was no true marriage between Mary and Joseph. For Jerome says against Helvidius that Joseph "was Mary's guardian rather than her husband." But if this was a true marriage, Joseph was truly her husband. Therefore there was no true marriage between Mary and Joseph.
Objection 2. Further, on Matthew 1:16: "Jacob begot Joseph the husband of Mary," Jerome says: "When thou readest 'husband' suspect not a marriage; but remember that Scripture is wont to speak of those who are betrothed as husband and wife." But a true marriage is not effected by the betrothal, but by the wedding. Therefore, there was no true marriage between the Blessed Virgin and Joseph.
Objection 3. Further, it is written (Matthew 1:19): "Joseph, her husband, being a just man, and not willing to take her away [Douay: 'publicly to expose her', i.e. to take her to his home in order to cohabit with her, was minded to put her away privately, i.e. to postpone the wedding," as Remigius [Cf. Catena Aurea in Matth.] expounds. Therefore, it seems that, as the wedding was not yet solemnized, there was no true marriage: especially since, after the marriage contract, no one can lawfully put his wife away.
On the contrary, Augustine says (De Consensu Evang. ii): "It cannot be allowed that the evangelist thought that Joseph ought to sever his union with Mary" (since he said that Joseph was Mary's husband) "on the ground that in giving birth to Christ, she had not conceived of him, but remained a virgin. For by this example the faithful are taught that if after marriage they remain continent by mutual consent, their union is still and is rightly called marriage, even without intercourse of the sexes."
I answer that, 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 (Matthew 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 Luke 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."
Reply to Objection 1. Jerome uses the term "husband" in reference to marriage consummated.
Reply to Objection 2. By marriage Jerome means the nuptial intercourse.
Reply to Objection 3. As Chrysostom says (Hom. i super Matth. [Opus Imperfectum among the supposititious works ascribed to St. Chrysostom) the Blessed Virgin was so espoused to Joseph that she dwelt in his home: "for just as she who conceives in her husband's house is understood to have conceived of him, so she who conceives elsewhere is suspect." Consequently sufficient precaution would not have been taken to safeguard the fair fame of the Blessed Virgin, if she had not the entry of her husband's house. Wherefore the words, "not willing to take her away" are better rendered as meaning, "not willing publicly to expose her," than understood of taking her to his house. Hence the evangelist adds that "he was minded to put her away privately." But although she had the entry of Joseph's house by reason of her first promise of espousals, yet the time had not yet come for the solemnizing of the wedding; for which reason they had not yet consummated the marriage. Therefore, as Chrysostom says (Hom. iv in Matth.): "The evangelist does not say, 'before she was taken to the house of her husband,' because she was already in the house. For it was the custom among the ancients for espoused maidens to enter frequently the houses of them to whom they were betrothed." Therefore the angel also said to Joseph: "Fear not to take unto thee Mary thy wife"; that is: "Fear not to solemnize your marriage with her." Others, however, say that she was not yet admitted to his house, but only betrothed to him. But the first is more in keeping with the Gospel narrative.
Source: Summa Theologica (New Advent)