Disclaimer: I'm a PP employee.
While it's complex (and I don't fully understand it all myself), the simple answer is risk management / fraud. Adult content tends to be extremely risky from a fraud and legal compliance perspective and companies like PayPal (and Stripe, and Google Checkout, among others) choose not to do business in this arena because the risk / reward model just doesn't justify it.
Specifically, subscriptions to sites that offer digital adult content are not allowed per ToS: