If something is obscene, the government can regulate it, and criminalize its use. And that was its biggest problem. Unfortunately, Congress confused them as well. That's why the CDA ruling is good news for free speech advocates, but makes things more confusing for parents. Long before the CDA was dreamed up, there were already laws on the books that outlawed pornography.
Parents have a lot of laws in their arsenal.
And laws that protect people on the ground almost always apply equally in cyberspace.