Because our kids are being thrown into the sex arena before they're ready. So many of them lack judgment, so many of them get into trouble, so many of them have their lives ruined. Content that an adult, because of their experience, because of the matter-of-fact way they can relate to sex, can send a kid in search of a partner.
Yeah, teens are naive. More than that, they're immature. You can't hand that to somebody on a platter -- it's something they have to discover for themselves -- but being privy to conversations between adults who know where to draw the line isn't going to help their judgement.