I disagree. Peer pressure, the media, curiousity, and raging hormones and a sex drive cause promiscuity in young people.
Sex ed only does good. As if anybody taking sex ed didn't know all the juicy details before? That kind of stuff you pick up outside the class room, its the safe sex info that you might not get anywhere else. Of course its not hard to find that information, but many kids won't bother.
The bottom line is that the US is undeniably backward in its regard for sexuality, especially with young people. Many other developed countries have fewer teen pregnancies and STDs even though their teens have plenty of sex and there is no sheltering of them. Hell when I was in Britian this summer there was full frontal nudity on BBC, a publicly funded channel! Much like alcohol control, we have problems with it because we make it a big deal. If it was just a part of life that people are gradually exposed to as they grow up, it would be less of a problem.
But in North America, sex is bad. Well guess what young people just LOVE to do? Rebel, do what they're not supposed to, be it listening to weird music, getting tattoos and piercings, or having sex when you're not really ready.