Valve confirms credit card companies pressured it to delist certain adult games

which is relevant how exactly?

merchants don't deal with mastercard, they deal with an acquiring bank

of which there are hundreds

no doubt one of which will be happy to take the business

Why do you think it's fair/acceptable to strongarm niche sectors that want to process credit cards just like everyone else?

Simulated "immoral" activity could be considered a moral gray area. If nothing else, morality is subjective.

So I think it's reasonable to argue for private, individual consumption of morally subjective material (not least of which is the logistical difficulty of preventing such things), as well as the right to create and sell such things. (You or I might approve of or oppose those things, but that's a different argument from what I make below.)

Aside from that, I don't think Valve or a payment processor is obligated to be a neutral party. Whether it might come from collective consumer backlash or whoever makes decisions for an organization deciding what they will or will not allow to flow through their system, I think they too should have the right to allow or ban things. If publishers and consumers want their morally gray content, so be it, but don't feel entitled to have Steam and VISA along for the ride if they don't want to be.

Hypothetically, Valve might prefer Steam be neutral, because money. But then they have the option to fight their payment processor or look for alternatives, rather than "forcing" their payment processor to be a part of something that the payment processor opposes.

TL;DR when a morally subjective issue involves a lot of parties, every party should have the right to "opt out" if they are morally opposed. (in my opinion)

I'm not sure I buy the chargeback angle. It's commonly trotted out as a reason card companies would enforce censorship, but it doesn't make sense with the actions they take. Chargeback fees are paid by the merchant regardless of the chargeback's success, and are supposed to cover the costs of administering it (and then some). The very selective rules applied here are pretty odd from that angle too, if adult content chargebacks/fraud is the issue, then all of it should be the issue, not small niches.

Fraud is likely more realistic of an issue, but that's probably an issue with games in general, not just adult titles.

There are already high-risk merchant accounts with higher fees and cash reserve requirements, but AFAIK companies like Valve aren't being given any options other than comply or be destroyed.

That's not true, anti-sex work and anti-porn activists have specifically been pressuring payment processors to assume these policies. The processors as the critical control point of this whole thing was identified decades ago and conservative christian think tanks have been pursuing this path since then.

This is part of a long-term plan to de facto ban lgbtq content without having to deal with first amendment protections. First have the payment processors ban explicit content, then have queer content categorized as explicit.