I think if Mary had said no...God would have simply found someone else. Simple. I don't think He would've smitten her or anything like that. He would have simply moved on and found another young woman.

But more to the point about the rape thing, I see what you're saying, but I don't see it that way because God was all knowing and omnipotent and he knew Mary loved Him and had a heart for God. So that's why I think he told her what He was going to do.

My 2 cents. :)