Sex isn't about babies? I mean, in our enlightened technologically advanced time, we have the luxury of making choices that include intercourse 'without consequences' (there's always consequences whether emotional or otherwise) but for the majority of the past this hasn't been the case. Most in the past would have always had to consider the possibility (as we still should) that there was a good chance that a child would be the result of their union. That was kinda the point, particularly if you're talking evolutionarily speaking.
Regardless sex has always included the need for responsibility, and the move to ensure everyone views it as a recreational activity no more dangerous or life-affecting than riding a bike (so wear your helmet and elbow pads, Jimmy) is what bothers me the most.
First I made a quick correction to my post, adding an extra "only" to this bit:
...we in Western society [...] have taught kids that sex is dangerous, dirty, shameful, and to be hidden. And only for straight married people, of course, only to make babies. But that's all a lie.
Separating sex and reproduction
Of course sex is to make babies—if that's what you want—and of course it requires responsibility because it has consequences. But for humans it has never been purely reproductive (in mammal species where that is the case, females go into heat when they're fertile, and aren't interested in sex the rest of the time); it's part of how intimate partners maintain their relationships, even if they're brief. Very few people who never want babies, or don't want them right now, forgo sex to avoid pregnancy. And I think that's not only good, but entirely feasible now.
Yes, until very, very recently, pregnancy (for straight couples, anyway) was a very likely consequence of sex. But it need not be anymore, because modern forms of birth control (and disease prevention!) are very effective when people know how to use them properly. So today, sex and reproduction can be separated—something people tried, and mostly failed, to do before the modern era—but only if the people involved are knowledgeable. The Pill and its progeny, the ubiquity and reliability of condoms, and the reduction of ignorance about sex—these have changed things, profoundly, and in my opinion for the better.
Unless you're, say, Lance Armstrong, sex is indeed generally more life-affecting that riding a bicycle, but it need not be more dangerous, and I think it's good to see it as a recreational activity too, because that's how our brains seem to treat it. When my kids learned to ride bikes, we gave them a few tips and let them go on their way to figure it out on their own, with the associated bumps and bruises. But I think sex is much more important than that, which is why they need to learn a lot about it than they did about bicycles, both before and after they become sexually active—something they will do at some point, likely not in too many years.
Comfortable, normal, expected—and better
My wife and I have tried to make their learning about sex something comfortable and normal and expected as they grow up, much like learning about earning and saving money, or being politically and socially aware, or becoming independent people more generally. The evidence seems pretty clear that sheltering them from that knowledge, especially now that they're soon to be teenagers, wouldn't mean they won't have sex or that they're likely to wait longer, just that they'll be less informed when they do, and likely to make poorer choices. Conversely, the more they know, the better choices they're likely to make.
I've been lucky in my sexual history that I haven't had any genuinely bad experiences. But do I wish I'd known more than I did, at a younger age? You bet. (And I knew an okay amount thanks to supportive parents and decent resources through school and elsewhere.) I don't think it would have made me more sexually active than I was, but I would have known more about what I wanted, and learned more about what my partners wanted, earlier, and maybe had even better experiences than I did.
And it's not all even about sex. My kids know people who are straight and people who are gay, and people who are somewhere in between. People with infants, kids, or grown-up children; and people who have no kids yet or plan never to have them. People in long-term, steady, monogamous relationships; single people; and people in considerably more complicated arrangements. While those relationships involve sex, that's not all they are about, and my daughters see how people can live all sorts of lives, and still be happy and fulfilled. That, I hope, will help them be open to figuring out what makes them happy and fulfilled, sexually and otherwise.
Being a parent is often trying to make things better for your kids than they were for you. I include whatever sex lives my daughters choose to have as one of the things I would like to be better for them.
And no, I don't want to know very much about it when they get there!