1. In Luke 1:34, when Mary was told by the angel Gabriel that she was chosen to be the Mother of the Messiah, she asked the question, literally translated from the Greek, "How shall this be since I know not man?" This question makes no sense unless Mary had a vow of virginity.
When we consider that Mary and Joseph were already "espoused," according to verse 27 of this same chapter, we understand Mary and Joseph already have what would be akin to a ratified marriage in the New Covenant. They were married. That would mean Joseph would have had the right to the marriage bed. Normally, after the espousal the husband would go off and prepare a home for his new bride and then come and receive her into his home where the union would be consummated. This is precisely why Joseph intended to "divorce her quietly" (Mt 1:19) when he later discovered she was pregnant.
This background is significant because a newly married woman would not ask the question "How shall this be?" She would know—unless, of course, that woman had taken a vow of virginity. Mary believed the message, but wanted to know how this was going to be accomplished. This indicates she was not planning on the normal course of events for her future with Joseph.
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3. Mary is depicted as the spouse of the Holy Spirit in Scripture. In Luke 1:34, when Mary asks the angel how she will conceive a child, the angel responds: "The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore the child to be born will be called holy, the Son of God."
This is nuptial language hearkening back to Ruth 3:8, where Ruth said to Boaz "spread your skirt over me" when she revealed to him his duty to marry her according to the law of Deuteronomy 25. When Mary became pregnant, Joseph would have been required to divorce her because she would then belong to another (see Dt 24:1-4; Jer 3:1). But when Joseph found out that "the other" was the Holy Spirit, the idea of his having conjugal relations with Mary was not a consideration.