He would not know how to reply because he doesn't know that ALL girls like rough sex. If you are the only girl that he's been with then he is maybe curious that he is missing out on something, but too embarrassed to admit that he's not that sexually experienced. If his prior experience of sexual relationships has been watching porn then he is going to be a bit disappointed with a normal relationship which from my limited experience is far from all that lip licking and bottom slapping.