"From the outside, it looks like this is a market failure that has no reason to exist other than institutional bias that is no longer grounded in reality."
I think you're being a bit naive there. Surely those companies would have no objection to tasteful nudity or educational content, but if you want to process actual porn you'll have accept a lot of seriously offensive content that you definitely don't want to directly associate with your brand.
Creating an entirely efficient market is a hard problem, so the existence of market failures should not be surprising. But, when we see a clear example of a market failure being introduced, we should question what is the reason, and if there is a way we can avoid it.
[0] Even after the 5% fees Patreon charges. There are also cases where the transaction is not entirely quid-pro-quo (eg. people donating to support the creation of free content), which mucks up the Econ 101 analysis, but I don't think effects the broader point.
The payment processor of Patreon has an interest not to deal with the porn, therefore declines the business.
The payment processor of competing actual porn platforms has a market niche to take exactly that business.
All the middlemen that are required here (for complex reasons) get paid for making it happen.
Nothing is lost, therefore no inefficiency, therefore no failure. The money not spent on one performer through Patreon can instead be spent on any other performer wise enough not to rely on Patreon for payment.
To be clear, there was a market failure before Patreon entered the scene because there was simply no market for these transactions to take place. But now, Patreon is deliberatly re-introducing that market failure. Further their reason for doing so is a generic excuse of "porn bad for payment processors"; not some specific "we noticed that the porn monetized on our platform results in an unacceptably high level of chargebacks". From the outside, it looks like this is a market failure that has no reason to exist other than institutional bias that is no longer grounded in reality.