Why do we not teach abstinence first in schools/sex ed classes? Only GUARANTEED birth control?

I don't understand why schools don't teach abstinence until marraige as a first (but not only) line of birth control. I find it interesting that we don't tell kids that condoms are only about 88% effective, while a number of other things can and often do occur that negate the usage altogether (condom breaks, slips off, etc).Why do we not teach abstinence first in schools/sex ed classes? Only GUARANTEED birth control?
That's propaganda, condoms are very effective, its the lack of consistent condom use that is the problem. Even so, they should be taught to use 2 reliable forms of contraception until marriage, not just one. Although I agree with religious conservatives who want to challenge the youth to delay sexual activity for a while, I totally disagree with their message that a condom is a provision for sin. I send my Christian friends to the website below which debunks the whole thing against premarital sex in the Bible. That way they have no excuse not to support comprehensive sex education.Why do we not teach abstinence first in schools/sex ed classes? Only GUARANTEED birth control?
My school did.

lip plumper