1. Define "porn"
2. Get every other TLD operator, including all the international ones, on board with your definition and rules (e.g. are we only banning websites on port 80, or would FTP sites also be verboten? password-protected sites? is classical art exempt?)
3. Enforce the censorship via some kind of magical system that is automated, doesn't trigger false positives, and allows for an appeal process
Yeah, that'll be a snap to put together.
Legislators will simply say that all sites that feature nudity in a non medical setting will have to be on the xxx domain name. This isn't merely just porn so it can be passed off as 'neutral' and not legislating morality but rather legislating categorizations ("bringing order to the Wild West of the internet").
Now as for how they force sites onto the .xxx domain name, the choices are:
1. Legislators then simply blacklist all sites that feature nudity in other TLDs. This could be called the "Australian" approach or the "Great Firewall of Foo".
2. Fine companies within your country that are running sites outside of the XXX TLD. If only the US and EU do this, it'll drive 98% of the money-making porn industry into the XXX TLD. Then maybe for completionism you can implement #1 against 'foreign porn peddlers'.
False positives are not a concern for legislators. If there's a worry about blowback, they'll simply make the blacklist confidential, and the appeals process non-transparent.
0. Repeal the First Amendment.
Moreover most, if not all, such "speech" (dissemination of pornography) will ultimately be commercial and so not come under the full auspice of the First Amendment protections.