Sean, I think the picture you're thinking of is this one

That's the one, Matthew - though I remember it a little differently but the tree does seem to be offering itself rather obligingly to be entirely natural.

How else do you separate sexual acts between men and women into two distinct component acts that be renounced individually?

I've no doubt that Mary was under no illusions as what would be expected of her as a wife even if she was an actual virgin, much less how far she could go with respect of maintaining that virginity as a mere technicality or otherwise. It is entirely reasonable, therefore, that a woman may know a man in any number of ways that are quite distinct from the ways a man may know a woman - a disctinction Mary is all aware of here with respect of her purity.