I have to say, btw, that my personal experience of underage sex (as in - I was underage - just - and my partner was a year older) is that it can be very good fun, with no angst and actually considerably less drama than a lot of the overage sex I've had!

My High School Sweetheart (tm) was a love, we had a lot of fun, and some quite good sex. We broke up with some teenage angst, but wound up still being friends eleven years later.

I guess by 'problems' you mean teen pregnancy? I agree that is a scary scary thing, but most of my friends were sexually active from their mid teens onwards and no one got pregnant. I think it's all about the context - teenagers shagging is part of growing up. I think shagging much older people, or where there's an abusive edge/uneven power balance it is much ickier. I must say - I've always been horribly freaked out by some of the Hermione/Snape stuff out there, even when there wasn't graphic sex attached!