But she didn't pretend she didn't know where babies come from.
The time-frame does matter. The timing of Mary marrying is sufficiently ambiguous so that some young girls in her position, as opposed to all young girls in her position, ... would ask such a question.
I find that dubious. Find me a young engaged girl. Tell her you've had a vision that she is going to conceive a child. Assuming she doesn't think you're mental, I bet she doesn't ask how this could be.
SD
I understand Protestant apprehension about the Immaculate Conception and the Assumption; however, I have never understood why perpetual virginity is a stumbling block for them.