For specifically sexually explicit stuff, it's because chargebacks are __significantly__ higher for these types of purchases. High enough that it messes with the credit and counterparty risk modeling that processors use. You can use your imagination to come up with many reasons these result in more chargebacks than normal purchases.

Theoretically, they could just split out "explicit" vs "normal" risk categories, but there's two top problems there: 1) it's just fundamentally a smaller-yet-way-more-annoying category than the rest of their payments, and 2) tons of your partners (banks etc etc) have blanket-banned for all of the above reasons.

So... here we are.