Well, there's the fact that not all pr0n is XXX or sex-related. Creating an .xxx or a .sex effectively excludes any sufficiently different fetishes or paraphilias that don't explicitly involve sex, such as inflation or transformation.
16 Sep 2L: I wrote this in response to a writeup that isn't up here anymore.
Here's what I think now:
.xxx and .sex, or similar things of their ilk, probably wouldn't be effective because they'll be too easy to block. People who want to run porn sites that college students can visit will just buy domain names under different TLDs.
My opinion (which doesn't belong strictly in this node, but is probably too short for its own writeup) is that instead of these silly hyper-specific TLDs like .museum, the Big 8 Usenet hierarchies should be used. Don't .rec, .comp, .sci, and .soc seem like better new divisions for today's web?
Also, major companies buy their trademarks in domain name form across every TLD, and it'd be more useful for them to have something like microsoft.news (which could be used for MS information) than microsoft.xxx; or disney.talk (which could be used for movie-related discussions) than disney.sex, now wouldn't it?