As somebody who used to work at an adult company (and has a different adult company as one of my clients), I'll say that most adult companies are probably excited about the idea of making it easy for parents to block these domains, but not excited about the possibility of being forced into using them (and the subsequent possiblity of ISPs blocking .xxx domains en masse). Some of adult sites' biggest customers are in countries where porn is illegal, and giving ISPs an easy way to block the entire industry would hurt adult sites even more than they are already.

On a side note, why hasn't a .app TLD been created yet?