Why do you persist in assuming that this (by which I mean a phase, while young, of widespread casual sex) is such a bad thing? Of course there are practical issues of disease and pregnancy, but I don't believe that's your concern.
As a counterpoint, I would state that, at least based on my teenage years, many teenagers are desperately unhappy because their bodies are telling them to be sexual, but society confuses them into believing they want to be in love. All sorts of inappropriate crushes arise, social akwardness because every interaction with a pretty girl boils down to "how do I make her fall in love with me", and early marriages (marriage pretty much before 30 IMHO, and I suspect cause of vast amounts of unhappiness). Not to mention varous unhappiness later in life as men at age 45 begin to wish they'd done more exploring earlier in life and wreck their marriages on a brief curious impulse ("what's it like to be with a blonde? to be with two women at once? to try anal?").
Of course you can provide various anecdotes about fscked up teenagers who are screwing anyone they meet because of a sense of anomie. And I can provide anecdotes about hookers who love their work, rake in a ton of cash and are saving it wisely, with a plan to leave to business at age 30. Anecdotes are not data, and until I see what I consider to be unbiased data showing that the negatives of this behavior outweigh the positives (especially considered over a whole lifetime), I can't see what their is to condemn and be worried about.
You think I have a moral objection. I don't. You write, "many teenagers are desperately unhappy because their bodies are telling them to be sexual, but society confuses them into believing they want to be in love." This is precisely the kind of thing I'm worried about. It's a confusion which they don't have the training (or help) to resolve. It depresses me that this confusion affects their sex lives, because it makes it much harder for them to get a handle on something extremely personal and important. I think it damages them, and I don't see that you disagree.
Belated and overwordy comment here:
http://sxchoc.blogspot.com/2004/06/luv-week-pt-4-humanities-education-vs.html