Content segregation by domain name just doesn't work. The big reason to have .xxx, .sex, or .prn around is to keep people out of them. But web proxies, Google, and The Internet Archive let even those behind web filters visit such sites--- even accidentally. Nor is their use easy to police, given the ease of registering a domain name. The rest of the world will probably still allow porn even if the U.S. doesn't--- are you going to prevent people from visiting .nl and .fr?

Labelling on the web page itself is just as effective (and just as easy to make mandatory), but doesn't require mucking about with naming. It's still a bad idea, mind you, but there's no need to introduce a new top level domain to deal with the issue.