There are plenty of porn friendly payment providers. They just cost a lot of money relative to "normal" merchants because of a few simple reasons:
1) Chargebacks -- people chargeback the shit out of these kinds of transactions. Significant other sees the bill, gets pissed / confused about the charge / whatever
and gets it charged back.
2) Fraud -- people use stolen CC's to pay for this kind of thing.
3) Porn operators are, as a whole, rather shady themselves and present their own set of risks to a payment provider. A lot of those chargebacks in #1 are actually valid chargebacks.
You could say "use bitcoin", which would solve the chargeback problem but do nothing for fraud and shady porn sites. If anything it would exacerbate the shady behavior because consumers would have no recourse against fraud.
In short, collecting payment for porn sites a tough, costly business.