Have you looked at the stuff I'm talking about here?
As to reasons, I agree with Neil Postman's theory: Sex (especially commercialized and commodified images of it) competes with juvenile personality development and academic skill acquisition. When it enters into kids' awareness at too young an age, it puts harmful pressure on kids and diverts attention from healthier pre-pubescent endeavors.
This is not to say prudishness and shame are good things. It's simply a bad thing for 11-year-olds to watch drunken pre-orgies. Same goes for pornography of the harder variety. All that is fine and good for grown-ups, and perhaps that includes late teens. But childhood, as Postman argues, is an adult-managed arrangement, the quality of which matters greatly to us all. MTV is nothing but a disaster for kids. Watch it for a few days, and see if you can argue your way around this.