Not exactly. Visa was named as a counterparty in a class action against Mindgeek for monetizing child porn on their website. They lost, and there have been subsequent class actions.

https://www.theguardian.com/uk-news/2021/aug/19/onlyfans-to-...

Ultimately the ban was undone in exchange for onlyfans limiting the type of content available on the platform. So effectively, payment processors dictate which type of sexual activities, performed by consenting adults, are OK to depict and sell. Why? Why do they have that power?

That was because the government was prosecuting Pornhub for breaking the law (rape with real people, unlike these games).