I'm not convinced that VISA and Mastercard are totally hands off about content and that Stripe and Paypal are going against their wishes to enforce content restrictions completely on their own.
https://old.reddit.com/r/Steam/comments/1mf7uei/steam_update...
Mastercard never talked to Steam directly. That means all the discussion about Mastercard (from Steams point of view) is practically hearsay.
Meanwhile, we have itch.io being VERY clear here that it's Stripe that has a no-porn policy. That gives us a DIRECT cause. And Stripe ALSO admits that they don't allow porn on their payment network.
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Has Steam even said who their payment processor is yet?
Steam definitely support PayPal in some areas, but their consumer terms and conditions don't disclose whether they use an intermediary for handling card payments.