It all comes down to their risk assessment of legal and/or illegal action from government.
Patreon is small enough to be ignored, so long as they don't push too much. I think that's why they allow porn (it's also the majority of their business at this point).
Credit card companies and Paypal are way too big to take any risk.
Paypal's no porn policy makes sense, in light of what the US Federal government has already done illegally in the past (
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My assessment is that Steam is worried enough of someone building a platform for porn games that would grow and then expand to non-porn, leading to competition. I honestly believe that's the only reason they allow porn.
But their review staff isn't allowing me to publish titles as early access, and requires a final build to even approve a listing as "coming soon", something they don't usually do. They've already rejected 3 of my attempts to get new titles in the store with BS reasons, with no possibility of appeal. So the company is okay with it but the staff seems to be pushing their own agenda.
Essentially there's only one reason these companies allow/ban porn: their assessment of impact on profits.