Mary, who was well aware of all of this history, nevertheless professes herself troubled and perplexed as to how this promised son could ever be engendered in her. Why would she not simply suppose that she'd get pregnant with her husband, just like the other holy mothers in Israel got pregnant with their husbands?
She reacted this way because, unlike the other women who knew they'd had sex with their husbands, Mary had not. Add in to this and you've got an angel telling her this. Hence her confusion on the "how" this would happen.
She understood biology and how to make babies.
She was a married woman. So were Sarah, Hannah, and Samson's mothers: married women. If Mary knew she would shortly be having sex with Joseph, if this was expected to be the natural course of things, there wouldn't have been any initial perplexity about where this promised baby in the unspecified future (as yet unconceived) was going to come from.
Look at it closely. You'll notice she was perplexed BEFORE she was told that the child would be the Son of the Most High. As far as Mary knew at that moment, it was a promised natural baby in the natural manner.
Like... WHAT? Me, have a baby? Sometime in the future? How would that happen?