I used to work for a payment processor. Our upstream bank would not allow us to handle any adult content at all. The justification used is that adult content is too risky because of chargeback rates.

Apparently the chargeback rates on adult content is really high. Anecdotally, it makes sense. You buy the nude pics, your husband sees the charge on your card, "I was hacked" boom, chargeback. You buy the nude pics, do the nasty alone in your room, have some regrets once the pressure to do the nasty is gone, try to get a refund, get refused, boom chargeback.

I'm not sure that quite holds up in our post-Pornhub world, where everyone is openly admitting to using porn these days. But banks are conservative (in all the ways) and slow to change.