I was just talking about this with my mom the other day. It's something that I've heard debated countless times while I was growing up, and everyone is always confused by it. Life can't exist without sex (yet), but it's shunned amazingly in the United States. No idea why. I can offer some guesses:
Violence doesn't have consequences--wait, stop! What I mean to say is, parents see this violent stuff all the time and figure it has no effect on the kid because they grow up around it. No big deal. Sex though, they don't grow up around it, and being exposed to it can lead to some...interesting conversations. Conversations that can lead to the loss of innocence of a child because they suddenly want to go out and try it. You don't have to worry about Timmy going out to try killing someone, but going out to try a get a look at Susie's breasts? Oh dear or dear. And the phone call from Susie's parents...?
Parents don't know how to talk about that 'stuff', so they sweep it under the rug and freak whenever it comes out.
Social stigma--Girls are sluts if they sleep around. Men and champions if they've slept around. Therefore, women get shamed to the point where no one wants to talk about it.
That whole having-a-baby-out-of-wedlock thing is still very much frowned upon. But again, only if you're the mother. Father, you made a poor choice, but people will eventually forget about it.
What you think is acceptable in terms of appropriate sexual ANYTHING isn't the same as what your neighbor thinks. Therefore, since we have become so worried about offending anyone, we just don't talk about it.
So basically, I think it boils down to the fact that we just don't feel comfortable talking about sex, and that discomfort has lead to us shunning it because we don't want to feel uncomfortable. Some of the best conversations I had with my ex girlfriend were about her thoughts on sex, what she liked, what she didn't like, and what she thought of other men she had slept with. I found it all very interesting, being a virgin myself, and it was nice to have someone who was finally willing to talk about all that stuff without getting uncomfortable or wanting to change the subject.