Any ISP or browser that did that as a default or mandetory aspect would be giving itself the kiss of death. I have no fears of that ever happening. As already mentioned by someone else, p0rn is the whole reason the internet got on this far this fast.
It would make it easier to patrol my kids at home, but then again, they have friends and I can't control what they see on their friend's computer.
I am willing to bet that churches would like to have their own tld. It would be no different than goverment agencies getting a .gov or schools getting a .edu.
But even beyond that, laws... Pshaw. Just take a look at the homepage today to see how effective laws are at regulating anything on the internet. A country could pass a law saying tehy had to be only on a x x x domain, but who is going to patrol it? I mean they are doing such a <sarcastic>fantastic</sarcastic> job patroling for kiddie p0rn and that is already illegal. What happens when there are millions of sites suddenly to patrol?
US courts are in place to protect first amendment rights. Heck, they just shot down the internet p0rn law that was passed years ago and has been in teh courts since then. You think that after doing that the courts would let a blanket block of a tld go through?
It would be just as easy for a state to block all sites that contain a certain word, but you don't see that happening, do you?