Something that seems to get lost in a lot of discussions on sex ed: sex ed is not just for children and teenagers. It's for future adults. It seems like there's a lot of talk about what a fourteen-year-old needs to know, and not about what a person will eventually need to know. A lot of statistics and speculation about teenage sexual activities and STDs and pregnancy, and not enough about these things in the general population.
High school isn't about learning to be fourteen years old. Most kids show up already knowing how to listen to terrible music and sleep until 3pm on weekends. Why teach a fourteen-year-old biology or trigonometry when the only job a fourteen-year-old is likely to have is burger-flipping? Because you're not teaching the fourteen-year-old; you're teaching the future thirty-year-old doctor or architect.
(Digression: I really wish schools did more to teach for adulthood in general. I'd gladly have given up a semester of Western Civ to have had a class on "how to find a job and an apartment" or "how to treat minor illnesses and when to see a doctor." There's a place for high-minded well-rounded education and I don't want to be one of those dicks who asks "when am I ever gonna need to know about stupid old ancient Greece?", but I'd gladly sacrifice a little bit of Greece to get in a couple hours of lecture and discussion on "how to tell if something is a scam.")
And sex, as it so often turns out, isn't some sort of special case. When you teach a fourteen-year-old how to put on a condom or what consent means, you're teaching a twenty-year-old. And a thirty-year-old. They may not look it right now, but when you teach sex ed to a bunch of immature teenagers, you're teaching college students, married people, parents, police officers, journalists, legislators, and sex ed teachers. You're telling them things they'll remember, and maybe still rely on, when they're a bunch of fifty-year-olds.
You wanna stick "all you need to know right now is abstinence" on them?
P.S.: "Train up a child in the way he should go: and when he is old, he will not depart from it." I didn't want to make the actual quote the title because I didn't like the "he" language but felt it would be artificial to remove it, but now you don't have to look it up.