Objection 1. It would seem that the Mother of God did not take a vow of virginity. For it is written (Deuteronomy 7:14): "No one shall be barren among you of either sex." But sterility is a consequence of virginity. Therefore the keeping of virginity was contrary to the commandment of the Old Law. But before Christ was born the old law was still in force. Therefore at that time the Blessed Virgin could not lawfully take a vow of virginity.
Objection 2. Further, the Apostle says (1 Corinthians 7:25): "Concerning virgins I have no commandment of the Lord; but I give counsel." But the perfection of the counsels was to take its beginning from Christ, who is the "end of the Law," as the Apostle says (Romans 10:4). It was not therefore becoming that the Virgin should take a vow of virginity.
Objection 3. Further, the gloss of Jerome says on 1 Timothy 5:12, that "for those who are vowed to virginity, it is reprehensible not only to marry, but also to desire to be married." But the Mother of Christ committed no sin for which she could be reprehended, as stated above (Question 27, Article 4). Since therefore she was "espoused," as related by Luke 1:27 it seems that she did not take a vow of virginity.
On the contrary, Augustine says (De Sanct. Virg. iv): "Mary answered the announcing angel: 'How shall this be done, because I know not man?' She would not have said this unless she had already vowed her virginity to God."
I answer that, As we have stated in the II-II, 88, 6, works of perfection are more praiseworthy when performed in fulfilment of a vow. Now it is clear that for reasons already given (1,2,3) virginity had a special place in the Mother of God. It was therefore fitting that her virginity should be consecrated to God by vow. Nevertheless because, while the Law was in force both men and women were bound to attend to the duty of begetting, since the worship of God was spread according to carnal origin, until Christ was born of that people; the Mother of God is not believed to have taken an absolute vow of virginity, before being espoused to Joseph, although she desired to do so, yet yielding her own will to God's judgment. Afterwards, however, having taken a husband, according as the custom of the time required, together with him she took a vow of virginity.
Reply to Objection 1. Because it seemed to be forbidden by the law not to take the necessary steps for leaving a posterity on earth, therefore the Mother of God did not vow virginity absolutely, but under the condition that it were pleasing to God. When, however, she knew that it was acceptable to God, she made the vow absolute, before the angel's Annunciation.
Reply to Objection 2. Just as the fulness of grace was in Christ perfectly, yet some beginning of the fulness preceded in His Mother; so also the observance of the counsels, which is an effect of God's grace, began its perfection in Christ, but was begun after a fashion in His Virgin Mother.
Reply to Objection 3. These words of the Apostle are to be understood of those who vow chastity absolutely. Christ's Mother did not do this until she was espoused to Joseph. After her espousals, however, by their common consent she took a vow of virginity together with her spouse.
Source: Summa Theologica (New Advent)