Martyrdom of Bartholomew

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Here is an interesting take on the Virgin birth story. An apocryphal writings on the acts and martyrdom of Bartholomew reads:

And as this virgin did not know man, so she, preserving her virginity, vowed a vow to the Lord God. And she said, I offer to Thee, O Lord, my virginity. And she being called for the salvation of many, observed this—that she might remain a virgin through the love of God, pure and undefiled. And suddenly, when she was shut up in her chamber, the archangel Gabriel appeared, gleaming like the sun; and when she was terrified at the sight, the angel said to her, Fear not, Mary; for thou hast found favour in the sight of the Lord, and thou shall conceive. And she cast off fear, and stood up, and said, How shall this be to me, since I know not man?

http://fam-faerch.dk/pseudigrapher/acts/martbart.htm
Here is Mary’s protest: How can this be? When the angel told her she was pregnant, the implication was she took a vow of chastity, meaning how can break my vow of being a virgin for life if I am to be a mother?