What do i say?

I kind of know this guy, but not too well. He's always thought i was beautiful and i always thought he was very attractive himself.. But today he asked me if we could.. you know.. smash, as he put it. I don't want to because i haven't before and he has MANY times.. How do i tell him i don't want to? i still want him to think i'm attractive though.. and i don't want him to hate me.