On the surface, it seems strange that nudity and sex are more taboo than violence and gore, despite the fact that we generally don't really mind if people are nude and having sex (in private), but we don't especially want people violently hurt anywhere (even in private). But I think this is the cause.
If a child sees nudity/sex on TV, it can bring up shameful mixed feelings for the parents, because we keep this side of ourselves hidden from children. Yep, mommy and daddy do that, but god forbid we talk about it. So we're all harboring this shameful secret. We're drawn to it, but we don't like the uncomfortable conversations it invites. "Yes, for God's sake we admit it, we're having sex.. but please don't tell our kids."
Violence, on the other hand, is something that most people have no trouble feeling unadulterated (hah!) condemnation for - so if our kids see it, we can point at it and go "That's bad!" without the confusing, mixed feelings.
Secondly, I think there are people frightened about losing the idea of a common definition of 'obscenity'. It takes courage to accept that someone's nasty fetish is actually perfectly harmless, and the revulsion has to do with your preferences. I mean, what's next? Are my kids going to start doing this? It'll be everywhere! Soon we'll be walking past rows of Poo Weekly in the 7/11. It's a lot easier to stand in a herd and point at the scapegoat. To those who are comfortable with their positions in the herd, this is an important tool. Witness how often politicians are discredited by sexual indiscretions.