It's really depressing... in short, pubescents and teenagers have about the highest and most uncontrollable sex drives of anyone in society, their knowledge about the powerful, natural, inevitable urges they're feeling and the behavior they inspire is (thanks to the so-called "morality" being forced on their educations) minimal and often full of outright errors... leading at worst to disastrous, life-crippling mistakes and at best to a lot of confusion and fear. I would guess that for many raised as abstinent Christians, those deeply ingrained negative feelings about sex last well into marriage, possibly for all their lives -- you can't just spend decades putting forth a massive internal effort fighting your overwhelming natural drives, and then suddenly say "Oh, well now it's okay and good and beautiful." It's like male race horses who have to be first conditioned not to mount female horses on the track, and then when they retire to stud, have to be "re-trained" to be able to mate... once you've been beaten (literally or figuratively) into knowing something is bad and wrong and to be avoided at all costs, you can't just switch it back on.
Further, teaching children that sex is bad -- that only naughty, sinful people have sex -- makes them not only feel needlessly shameful about their own unavoidable sexual desires and afraid to ask parents or anyone else vital questions; but, especially in the case of young girls, since they're told that what they "really want" is love and marriage, when they hit puberty and the hormones start churning such that what they "really want" is, in fact, sex, they understandably can mistake one feeling for another and make dangerously poor decisions as a result.
And you can't even argue about it -- anyone who wants minors to know about sex is obviously a family-destroying, life-ruining pervert. Judith Levine noted that just for suggesting that children SHOULD know about sex and that it isn't inherently evil or disgusting, she's been branded a child molester.
