Mary was a teenage girl that was being married off to an older man who was already widowed with children (at least, I think that’s the Catholic claim. “Perpetual virginity” and all that means that Jesus shouldn’t have siblings…)

I think rationally she was a teenage girl that was raped by a Roman soldier, and considering that not being a virgin on your wedding night was the kind of thing that, at least on the books, could get you killed…

If Mary lied, that was a really smart move.