There's no actual sex in this book, and the men don't even talk about it in very explicit terms. While Jimmy Cross does fantasize about tying Martha to his bed, it's not really in a super-sexual way – he just wants to touch the girl's knee. Weird, we know. But it probably has something to do with his interest in whether or not she's a virgin. He's really more obsessed with the idea of innocence in general than with virginity. So he wants to touch her knee because it looks like the knee of a virgin (whatever that means), and he wants to touch innocence.