Well, it's less that she can't, more that it would be a bit more complicated and also females are less inclined toward rape in my experience. And of course, she finds out kind rather unpleasant to look at. It just wouldn't be a good lay.
GhostDIGIT and Lycodrake's 7 by the way. Also, you've gotta admire this guy's guts, so maybe also add, "How is it that you don't quake in awe and terror before a dragon? Are you somehow familiar with my kind?"