4 Her virginity also itself was on this account more pleasing and accepted, in that it was not that Christ being conceived in her, rescued it beforehand from a husband who would violate it, Himself to preserve it; but, before He was conceived, chose it, already dedicated to God, as that from which to be born. This is shown by the words which Mary spoke in answer to the Angel announcing to her her conception; “How,” says she, “shall this be, seeing I know not a man?” Which assuredly she would not say, unless she had before vowed herself unto God as a virgin.
But, because the habits of the Israelites as yet refused this, she was espoused to a just man, who would not take from her by violence, but rather guard against violent persons, what she had already vowed. Although, even if she had said this only, “How shall this take place?” and had not added, “seeing I know not a man,” certainly she would not have asked, how, being a female, she should give birth to her promised Son, if she had married with purpose of sexual intercourse. She might have been bidden also to continue a virgin, that in her by fitting miracle the Son of God should receive the form of a servant, but, being to be a pattern to holy virgins, lest it should be thought that she alone needed to be a virgin, who had obtained to conceive a child even without sexual intercourse, she dedicated her virginity to God, when as yet she knew not what she should conceive, in order that the imitation of a heavenly life in an earthly and mortal body should take place of vow, not of command; through love of choosing, not through necessity of doing service.
Thus Christ by being born of a virgin, who, before she knew Who was to be born of her, had determined to continue a virgin, chose rather to approve, than to command, holy virginity. And thus, even in the female herself, in whom He took the form of a servant, He willed that virginity should be free.
Source: On Holy Virginity (New Advent)