Disclaimer: I worked for a payment processor that handled a large portion of adult site traffic.

Payment processors categorize porn sites as a high-risk businesses because of the higher likelihood of chargebacks from customers who spot such a transaction on their statement. The processing, upfront, and recurring fees are higher than normal merchants to factor in this risk, and one or more card networks have explicit rules on which content is acceptable for purchase (guided mostly by laws against bestiality and the scenarios in which urine and feces are presented).