You don't have to have PornCoin, they can use an exchange like Binance which has a lot of different currencies supported, and the customer could pay in any one of these. Or they could use Request Network if that pans out.

And lastly, Bitcoin fees currently aren't large and you can get a confirmation with a few cents fee. That might stop being so if it becomes too popular again, but if off chain solutions like Lightning Network pan out, then that'd be sorted out.