Well...
Depends on the flexibility of the system and what people are willing to do. Some amateurish ideas follow. :-)
An easy, if not so pretty, solution would be to append .xxx to the current existing addresses, in a grandfathering system, so to speak. So, in the examples you gave, those sites would become hotsex.com.xxx and hotsex.net.xxx, but any newly registered site would not need (or be allowed?) to do that. The old addresses could redirect to the new .xxx appended ones for a while, say. After a certain period (a couple years? more?), the old addresses would stop working definitively.
People *already* don't follow the underlying logic anyway, it seems to me; why is El Reg's URL ended in .co.uk, while if the site was American it would be .com only? Brazilian sites are .com.br and so forth.
On the other hand, this could create new opportunities for scammers, typosquatters, people like that. But pornland is hardly known for being a safe corner of the interwebs to begin with, so I don't know how much worse it could get...