At what level of government? The state? The city?
Why can't a state like Utah ban whatever they want?
Certain things are constitutionally protected. Porn is, but obscenity isn't. The problem is that SCOTUS came up with the concept of community standards before the internet existed. If a website in San Francisco depicting gay porn is viewed by people in Alabama, what community standards should be used to judge whether the pornography in question is actually obscene? The prosecutor would want Alabama standards to apply, the defendant would want Bay Area standards.
Well, there's that Constitution thingy.