Chris, WHY do you think that having sex before 17 is so terrible? Even if they regret it later, teenagers have to make decisions for themselves sometimes. You said yourself that teenagers' bodies are built for it, and that's their mindset. Even if teens make poor decisions at the time, it's a part of growing up. People can make mistakes, but people can also have healthy relationships that involve sex before they're 17 as well.
I didn't say it was so terrible...but let's be honest: in most cases, it's a bad choice. And yes, sometimes they do have to make decisions for themselves: but what has that got to do with anything? Making decisions for themselves, even if necessary, doesn't mean we must APPROVE of those decisions.
Now, understand that I am religious, and as such, I think sex before marriage is basically always a bad idea. However, setting my Faith aside, I think it's still a bad idea, with SOME exceptions...but I think you and I both know those exceptions are rare. Usually it's an impulsive thing done out of hormones, and not out of thought, or love. It's lust.
Showing sex in movies isn't encouraging anything, it's just showing sex. It has to do with the mindset of that teenager, how he/she was raised, and ultimately what kind of person they are. You can show people whatever you want, how they react should be based on these factors.
You said yourself that seeing violence "demystifies" human suffering...so why doesn't it work both ways? Doesn't sex on screen demystify sexual promiscuity?
And yes, maturity is what it's ALL about. You said, as I quoted above, that I myself said that their bodies are basically ready...and you're right! But that doesn't mean their minds are. I have a theory, which I've shared before here, that if a child is raised properly, they'll be ready to become an adult just as puberty is ending...ready to work for a living, and raise to get married, have sex, and raise a family. However, today's society has decided that you're not an adult until you're 18/21...long after most people have become physically mature. IMO, physical maturity and mental maturity should, if nurtured properly, evolve at the same rate, and complete their cycle at the same time. It all comes back to lack of adequate parenting in today's country.
So yeah, I see sex in movies, and I'm not going to be effected by it...but we both know a lot of people will. A lot of people are raised in such a way that they are very sensitive to suggestions like that. We just disagree on how to go about it, I suppose.

But yes, in a world where almost all children were well-raised by their parents, we wouldn't have to worry about most people being exposed to this.
Giving out condoms and having a proper sex education curriculum aren't encouraging sex, they're teaching about it. What it sounds like you're saying is that sex before marriage is an unnatural, unhealthy thing, and that we should teach abstinence instead. If this isn't idealism, I don't know what is.
Why is abstinence idealism? Idealism would be assuming that everyone will become abstinent...but saying that teaching abstinence will HELP is perfectly realistic. There's a subtle, yet crucial, difference between the two.
Teaching about it? When you put a condom on a banana, you're practially simulating it. I have no doubt that many take this, conciously or otherwise, as a sense of authoratative approval of teenage sex...after all, they're showing you how to do it in full view of everyone! Obviously they don't mind.
Teaching sex, in THAT way, is definitely encouraging it. Who's more likely to have sex...a person who's a bit nervous about it, and doesn't know what to expect, or a person who knows what to expect, and knows how to have sex well?
Now, let me take this time to point out that I am ALL FOR making kids comfortable with sex...just not comfortable with sex at their age. I'm comfortable with it...my dad and I have talked openly about it several times, and I no longer shy away from it. In this household, we teach young children about sex very early, because they ought to know. They ought to get used to the concept, and be ready for it...but that's a very different thing from pretending it's okay for naive people acting on their impulses to have sex, and perhaps even produce a pregnancy. I will surely be teaching my kids about sex early and often...I just won't be encouraging it at such an age. When with a spouse, sex is amazingly beautiful...I believe it is a gift from God, to us. It's the ultimate expression of love, which is why it's a shame to see it tainted by people engaging in it for lust alone. That isn't love...and we want our children to see it as an act of love.
Like I said, though, if children were raised properly, they could get married younger, and relieve those sexual tensions. That's why we have these problems: we have people ready to have sex, who are mentally irresponsible in many ways. They need to get rid of all that sexual frustration, and today's society doesn't approve of them getting married to do so properly, because they're just not ready, mentally. It's a shame.
Ironically, that ties into the condoms: yes, I'll bet it cuts down on teen pregnancy in some ways...but in others, it might increase it, simply because more people will have sex in the first place. I DO, indeed, see the argument for it as a logical one...just not one I think makes sense in the end. I think right is right, and that showing kids these things only encourages more people to have sex at that age. Like I said, just because we can't stop them all, it doesn't mean we should start encouraging them. Just because some will be doing it no matter what, it doesn't mean we should pretend that it's all okay, and that it's inevitable that ALL people will.