Let’s be clear about what you mean when you say “outside the mainstream”, because that innocuous turn of phrase is doing a lot of work to cover what you’re really saying.
Visa and MasterCard disallow content depicting CSA, rape, incest, bestiality, necrophilia, scatological erotica, torture, extreme sexual violence, and revenge porn.
Equivocating child sexual abuse with the dude drawing consentual fan-fic or furry porn feels disingenuous at best, and more like bad faith though.
Because you're not wrong, the mainstream is fairly narrow, but to say the credit card duopoly excludes only the most heinous and vile imagery that can only barely be covered under "artistic expression" isn't exactly an entirely accurate depiction of reality.
I believe that happens for other reasons though. No law is telling Visa/Mastercard to prohibit payments to furry artists. They have some risk model that says it’s not good business and additionally pressure from advertising partners to not have their logo near that stuff.
Visa and MasterCard disallow content depicting CSA, rape, incest, bestiality, necrophilia, scatological erotica, torture, extreme sexual violence, and revenge porn.