But the article attributes the opposition to conservative pro-family groups.
I would favor the idea enthusiastically if there could be some way to require that all pornographers to register under the .XXX domain name. As you said, that would give people an easy way to block porn sites.
I'm certain that such a TLD-based requirement would be struck down as unconstitutional, based on the previous rulings. But a law requiring porno sites to be restricted to a numerical range of IP-addresses might stand a chance. It could be a sort of IPv6 "red light district" that could be blocked efficiently.