You could hack the system by only using a .xxx domain for your personal emails when dealing with the state. Tax man: "we can't send you that legal notice because our software blocks that domain" me: "not my problem"
To me, this makes the case of people that want real, imposed censorship weaker, because you can say, "Just block xxx and be done with it". With .com, they can say that someone needs to protect you because you might accidentally go to a bad .com website or something like that, so active filtration is needed. If .xxx becomes the standard and the .com just redirects, it will be easy for people who don't want porn to not get it, people who do want porn to get it, and people charged with filtration to relax a little bit.
Why is this bad? How does it strengthen the argument that the internet should be actively censored?
Just because .xxx exists doesn't mean there won't be "pornography" or "offensive materials" on .com/.{foo} addresses. This just provides an excuse for policy and law makers to write legislation saying "Pornography can only be on *.xxx domains".
It's naive to think this will have any effect whatsoever on the availability of porn to your average < 18 year old anywhere in the world.
It will have some effect, I think. I think that most reputable pornographers, in an effort to demonstrate compliance with standards and keep the adults-only vibe that keeps them on the right side of public opinion, will eventually migrate to xxx. It will be easy to block at least some pornography, which isn't to say that a filtered individual can't just go off to a known .com site. Porn will always exist on .com. It does help to some degree, however.
I don't expect the government to pass a law regarding xxx and even if it is passed I don't expect it to be remotely enforceable. .xxx is merely an opportunity to make life easier on everyone, if the standard is widely adopted.
It's also not entirely about filtration. People that want porn can peruse xxx domains and use scripts to highlight and/or prefer links to xxx.
The point is, .xxx draws a line, and there is quite a bit of disagreement as to what should be on which side of that line.
"I block .xxx but I still saw {thing I don't like}! They should be required to use .xxx!". "I need to access {important_resource.xxx} for {reasonable reason} but my school/university/ISP/telecom/church blocks it!
(Not to mention the whole can-of-worms that is inter-national law. Consider the differences in policy between countries like Saudia Arabia, Sweden, and the US.)
This is all dependent on good adoption of xxx of course, but if the TLD doesn't exist it's not even possible. So we're happy the opportunity is presented, and I think that mainstream pornographers that want social tolerance will prefer the xxx TLD as a means to illustrate their good intentions.
If enough legit stuff starts appearing with an .xxx domain name then there will be pressure to get rid of complete tld blocks.