@daloon: I don’t think it’s a hidden agenda. I think it’s a really naïve or really optimistic view of human nature.
The parents insist on no sex education, or on sex education based only on abstinence. They do this because they really think that abstinence until marriage is both morally correct and healthy, and because they think (incorrectly) that sex education classes and talks with parents are how their kids get their information about sex. So if the only message they send is that abstinence until marriage is the only way to go, then obviously their children will hear that message and heed it. Also, they don’t want to suggest to their kids that it might be acceptable to have premarital sex under any circumstances, and so they don’t want discussions of safe sex practices or birth control—because safe sex practices to prevent the spread of disease are irrelevant when both parties are a virgin on their wedding night, and birth control is irrelevant when you’re married and you want kids—especially if you aren’t aware that sex without pregnancy is a viable option. And finally, if you have an honest, science-based discussion of human sexuality, you have to touch on homosexuality—and that’s a very loaded subject.
There are many problems with this approach to sex education – but I think the people who do it are naïve and optimistic, not secretly plotting to overpopulate the world by feeding misinformation to their children. The first problem is that kids are getting messages about sex from all over the place, not just from the parents, and so when the parents’ message about abstinence is not consistent with all the messages they’re seeing in the entertainment media about how wonderful sex is, they distrust their parents’ message. But at the same time, very few people in the entertainment media are making a fuss about pregnancy and STIs, or about condoms and dental dams.
Maybe this level of optimism falls under “stupid” in your world. I don’t think people who believe that way are stupid; it’s sufficient for them to merely be wrong, as far as I’m concerned. And I can see how someone with good intentions but a few wrong ideas (such as the idea that homosexuality is a choice, and that teaching kids about the likely consequences of sex will encourage them to experiment more than not teaching them anything but letting them absorb the messages about sex that are out there) could, with lots of good intentions, come to the conclusion that sex education is a bad thing overall.