Welcome to JWD!
Well, I am a freak of sorts. I was married and divorced 3 times as a JW. So in order, here is the premarital behavior:
1st - Started having premarital sex 3 months before wedding, oral way before that.
2nd - Were good little boy and girl until wedding night.
3rd - Oral only.
So yes it does happen. I would guess the majority do, and never confess.