Government regulation of any sort of speech that's merely offensive to some (ignore for the moment such things as death threats, etc. which obviously should not be protected) is a huge mistake. You then get into free speech issues and dicey regulations that don't achieve what they are meant to.
For instance, lots of fine art has pictures of nude females in it. A Mormon or a Muslim may find that very offensive, while I as an agnostic may find it a tasteful example of art. But, let's say the government regulation says .porn is mandatory for all sites that "Display nude photographs or representations of persons in any form." Right there, all of that art has been shuffled off into an area that many people are unwilling to go.
High-school student Mike comes along and is researching Picasso's paintings and wants to find a complete gallery of Picasso's art to study the similarities between them. However, no full gallery is possible under a .com/.net/.org site because more than a few of Picasso's paintings are of nude men and women, and therefore relegated to .porn. And Mike's parents, thinking they're protecting their children from only "evil" sites, have banned access to that TLD. That information is basically banned from anyone who is restricted from accessing an arbitrarily-defined TLD that the government has decided upon.
Also note that such things as sex education and sites for teenagers confused about their sexual orientation could likely be shifted off to .porn as well. Never underestimate the power of certain members of Congress to push their extremely conservative views into laws simply because they want to get re-elected in Texas or Utah (to name just two). It's easy to say that porn sites should be put in their own TLD, but when you look at the difficulty and differences in deciding what "porn" is, it becomes a very dicey issue.