How are the words of Mary in Luke 1:34 incontrovertible proof of her perpetual virginity? Based on this question it seems that a lot of weight is given to the simple statement by Mary to Gabriel that she is a virgin. 


  Luke 1:34 NIV How will this be,” Mary asked the angel, “since I am a virgin?” 


Various translations use different phrasing here like “I do not know a man” or “I am not knowing a man”. Catholics make a big deal out of this verse declaring it to be proof that Mary was informing Gabriel that she was a woman who had vowed to remain a virgin forever.

What is the reasoning for making this seemingly unexceptional statement  into a cornerstone of the unsubstantiated (in scripture) dogma of the Perpetual Virginity of Mary?
One would think Mary knew the birds and the bees. One would think that she knew that after betrothal, a couple would cohabitate and consummate their marriage. Naturally, a baby would ensue approximately 9 months later. After all, that’s how Mary was born! Her mother and father went through the same process. One would think Mary knew that.
This is a translation of Gabriel’s statement to Mary:1

31 And behold, you shall conceive in your womb, and you shall give birth to a son, and you shall call his name “Jesus.”

At that very time, Mary was already betrothed to Joseph. It would be expected for them to cohabitate and consummate their marriage. Would it not? If indeed this were to be the case with Mary and Joseph—the consummation—then why would Mary ask Gabriel:

34 ...How shall this be since I do not know a man?

“To know” is, of course, a euphemism for “to have sex.” Thus, we could just as well translate that verse as such:

34 ...How shall this be since I do not have sex with a man?

Hello?! Mary?! Don’t you know what shall happen when you and Joseph consummate your marriage? What do you mean, “How shall this be?”
So, if you do not believe that Mary was going to remain a virgin perpetually, then you have to believe Mary did not understand what happens when a husband and wife consummate their marriage. You have to believe Mary did not understand the birds and the bees although her parents already handed her off to be married to another man. You have to believe that Mary was asking Gabriel for a biology lesson.
Note, Gabriel does not say to Mary, «μέλλεις συλλαμβάνειν»—“you are about to conceive,” implying that her conception was at hand, prior to her consummation with Joseph. Rather, he uses the future tense, and Mary, already being betrothed, would have rightly understood it as being fulfilled when she consummated her marriage to Joseph—if indeed she did not have a perpetual vow of virginity.

Footnotes
1 Luke 1:31: «καὶ ἰδού συλλήψῃ ἐν γαστρὶ καὶ τέξῃ υἱόν καὶ καλέσεις τὸ ὄνομα αὐτοῦ Ἰησοῦν»
2 Luke 1:34: «εἶπεν δὲ Μαριὰμ πρὸς τὸν ἄγγελον Πῶς ἔσται τοῦτο ἐπεὶ ἄνδρα οὐ γινώσκω»