It’s a bit of a mix. Mastercard/Visa do set some policies around this, but only due to, quite frankly undemocratic, political pressure. There been a few documented cases of particularly puritanical US politicians sending letters and making arbitrary public claims to “embarrass” Mastercard/Visa into restricting certain types of perfectly legal commerce. The impact of these policies is a bit arbitrary, as Mastercard/Visa generally aren’t in the business of restricting commerce (and thus their cut of the profits). So they tend to have short lived, but high impact, consequences on specific individuals or groups.
Really though, the primary reason why a company like stripe don’t want to be involved with these types of business, is the very high levels of fraud and chargebacks that come with the territory. Turns out people get embarrassed about porn appearing on their bank statements, and often put in dubious chargeback claims. Not to mention many banks have their fraud controls set to a hair-trigger for anything porn related.
The end result is processing these transactions is normally very expensive and high risk, due to the fraud and chargebacks. Which in turn put you at high risk of being kicked of the Mastercard/Visa networks. Mastercard/Visa mostly don’t give a shit what you’re selling, as long as you pay your dues. But they do get very upset when it looks like your business might threaten the perceived safety of credit/debit cards. As usual, protecting profits is treated much more seriously, than preventing any perceived moral failing.
As for governments, they officially don’t care. Selling porn is perfectly legal in the western world, so it only individuals in government who choose to abuse their positions to enforce their personal moral code on others (beyond what the law requires) that creates any kind of government “policy”.