This is the toughest of all entries to deal with on this list because I'm standing against thousands of years of established religious doctrine and calling it bullshit. Sorry, Jesus.
Many people view sex and sexuality in various forms as sinful and taboo and wrong. Look how many fundamentalists just cringe at the idea of sex ed being taught in schools. Counter this with the amount of people in the world who think you can't get pregnant if you have sex in a pool, or that sex with a virgin cures AIDS, and you'll see the value in education. But many people have a standpoint that if children are taught about it, they'll do it, as though sex ed were an instruction book that pressures you into doing things, in much the same way math class forces kids to become mathematicians.
Many religions view sex outside of marriage as morally wrong, or sex for non-procreative reasons as wrong. Gay sex is a sin, oral sex is a sin, pornography is a sin, and anything fun or different, or not missionary in the dark for the purpose of having a baby, is wrong. There's no good reason for it, of course, and the best answer you may get is that the Bible, or whatever religious text, speaks against it. Well, the Bible warns against wearing clothes of mingled fabric as well, so let's not pick and choose what words of God we can follow and which we can ignore.
The fact is, sex makes people uncomfortable in much of Western society because we've demonized it. And it's for no good reason at all. You can go back historically and you'll never find a good reason. Reasons, yes, but nothing good or defensible. Stupid reasons. The outcome of this is an abundance of hatred toward gays, an abundance of teen pregnancies, and an abundance of sexually transmitted diseases. That's how stupidity deals with problems. So don't be stupid and view sex and sexuality as bad. If it was bad, it wouldn't be the method by which life is created, for fuck's sake. That's a pretty big deal. And just like you don't always have to eat to stave off starvation, you don't need to bust out the sex swing just to reproduce. It's pretty obvious sex is meant to be fun. You stuck up folks need to chill the hell out.
For more ways we obsess over sex, check out If Sex Was Used to Advertise Everything.