if government etc would want to block porn, they could have done so already a long time ago, look at countries like China, Dubai... where adult content has been blocked successfully since many years. Of course there are always some adult content websites which can be reached, but the majority can be blocked easily, it doesn't need a .xxx extension for that.
The adult industry will register some keywords and their own "brands / .com domains", other than that, I doubt that they will push the extension or even use it in marketing campaigns.