Things like the .xxx proposal and opposition to net neutrality are steps towards the same kind of 'clean, curated experience' you get on the iPhone today.
Thankfully they seem to be getting knocked back every time they try, but only with great expense and effort from groups like the EFF. And they keep on trying new ways to ruin the Internet all the time.
See how well they're doing stopping piracy of normal media? They'll do even worse stopping pornography. There's nothing to fear from a .xxx domain.
But how much harm could they do in trying? That is the question you should ask. Look up Anthony Comstock if you want an example.
How certain are you of this? Would you be able to successfully defend every photograph or email you have ever produced against obscenity charges? I think, if it came down to it, the courts could find a hell of a lot of inadvertent pornographers out there.
Note I said 'could': Selective enforcement of over-broad laws is a boon and a horrible threat, because it implies everyone is guilty of something, so the government could pound anyone who bothers them if it chose to.
At first glance, it seems like a great idea, but as soon as you get into the details it's a nightmare. What .xxx and similar proposals mean is that someone, somewhere, possibly in another country and with values wildly at variance with yours, gets to censor the Internet for you. It's not a service I consider worth having.
Smarter people than I have written about this before: http://tools.ietf.org/html/rfc3675