I only provided the
link regarding the explanation of both a tradition of
perpetual virginity and references to Jesus' brothers in scripture, but as you mention it Adrian:
The following I find highly unconvincing (specifically that the question only makes sense given a vow of virginity)
"When the angel Gabriel appeared to Mary and told her that she would conceive a son, she asked, "How can this be since I have no relations with a man?" (Luke 1:34). From the Church’s earliest days, as the Fathers interpreted this Bible passage, Mary’s question was taken to mean that she had made a vow of lifelong virginity, even in marriage. (This was not common, but neither was it unheard of.) If she had not taken such a vow, the question would make no sense."
In saying this, you agree with my (Catholic) New Jerusalem Bible, which states that "[n]othing in the text suggests a vow of virginity", which I agree with in turn. I also agree that saying that it makes no sense without a vow is going too far. On the other hand:
In the sixth month the angel Gabriel was sent from God to a city of Galilee named Nazareth, to a virgin betrothed [b] to a man whose name was Joseph, of the house of David. And the virgin's name was Mary. And he came to her and said, "Greetings, O favored one, the Lord is with you!" But she was greatly troubled at the saying, and tried to discern what sort of greeting this might be. And the angel said to her, "Do not be afraid, Mary, for you have found favor with God. And behold, you will conceive in your womb and bear a son, and you shall call his name Jesus. He will be great and will be called the Son of the Most High. And the Lord God will give to him the throne of his father David, and he will reign over the house of Jacob forever, and of his kingdom there will be no end."
And Mary said to the angel, "How will this be, since I am a virgin?" [d]
And the angel answered her, "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.[...]" - Luke 1:26-35
[b] That is, legally pledged to be married
[d] Greek since I do not know a man
It is a bit of an odd question. This girl is pledged to be married, but the question seems to indicate that she has had no introduction to the facts of life, and more specifically, that marriage to Joseph might just possibly entail making babies. Why is she surprised at the idea that she is going to have a child, given that she hasn't been told about the
virgin birth at this point? I just realised that I could have posted what the article said after the bit you quoted - may as well now:
If she had anticipated having children in the normal way [...] she would hardly have to ask "how" she was to have a child, since conceiving a child in the "normal" way would be expected by a newlywed wife.
The text doesn't imply a vow, but a vow does provide an explanation for the text. It's a hypothesis which seems to fit the facts of the matter, rather than a proof, which happens to accord with a very venerable tradition. Feel free to explain it another way. I'd be quite interested; I don't think I've seen an attempt to explain it without a reference to the vow hypothesis.
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