Well don't I feel like an uninformed dumbass. Talk of standards aside, pornhub apparently includes the following header if you visit it.

rating: RTA-5042-1996-1400-1577-RTA

RTA stands for "restricted to adults". So the large websites, ie the entities that would actually be bound by any proposed legislation, are already proactively facilitating network operators and parents filtering them out. And apparently have been all along. Wild.