I think teenagers are physiologically disposed to try to have as much sex as possible. I might qualify that with "male teenagers" but since I'm not a female nor have I interviewed many people on this issue, I couldn't say whether women in general are any different.
Recalling my days as a sex-starved teenage male, I can say it took up a significant portion of my thoughts. I understand when some people say that sex is a driving force in human culture (expecially in cultures where sex is restricted and generally unavailable).
These days, though, it isn't such a big deal. It's like water: you think about it all the time if you're dying of thirst, but if you have more than you can drink you stop obsessing over it and you can think about other things.
So I think reports of high rates of teenage sex are a combination of desire for sex, whether it's happening or not, and obsession about sex, because it's generally not happening enough to fulfill the desire.
The first is biological, and unless someone puts forth an argument about industrial pollutants changing our hormonal balance then that hasn't changed in centuries or millennia.
The second relates to the first, but is mental, and may be affected by societal norms. A sex-focused media environment could make people think about sex more often. A sexually restrictive culture could make people obsess about it more (because they don't get it as much as they naturally want) and also make them feel guilty about wanting it at all.
Perversely, the common sentiment in the US is that sexually promiscuous men are "studs" and viewed positively, while sexually promiscuous women are "sluts" and viewed negatively. It seems that the obvious outlet would be male homosexuality, but that's viewed even less tolerantly than female heterosexual promiscuity. So instead, the prevailing sexual opinions serve to just make everyone feel bad ... except heterosexually-active males, of course. That seems to be a common pattern in norm formation, where the socially dominant group will place itself as the "normal" or "standard", and also as the "good" group, relegating all other groups to less-accepted positions. Strikingly, these "pariah" groups often accept the status quo and actually reinforce it among themselves! Of course, when I say this is the common sentiment in the US, it will of course be different regionally and in subcultures or specific social groups, and of course individuals believe wildly varying things. But it seems to be the general climate of sexual approval / disapproval.
And yes, there are a heck of a lot of sexual outlets that are outright illegal in many parts of the US (sex toys, videos and magazines, prostitution, strippers, sex clubs, etc). Yes, there are states where it is illegal to buy/sell/possess a "personal massager". People, this is a motor that's mounted slightly off-center and encased in plastic, and it's illegal to use one strictly because it makes you feel good. It doesn't involve anyone else but the battery manufacturers. Kind of strange.