Interesting point. I had a look. Indeed Payoneer and Stripe have very broad exclusion. Maybe there is fine print that I missed. They are corps. If it is legal, they probably have exceptions that are easy to miss. Since they deal worldwide they like to put it so broadly to cover their bases.
There should be an easy to use payment provider that only has the limitations of credit cards. Because, most of the stuff that is excluded by those providers can be done by the real payment providers (credit cards, banks). Including car rental, buying cigarettes and adult stuff. Especially adult stuff.
Anyways, the wording can be key, as you mentioned. Giving away the stuff for free, as in, not selling it. Making the payment not a payment for the game, but for other stuff, be it donation or whatever you wanna call it.
That being said, Paypal is the most lenient it seems to me. It is all so convoluted, they ban cigarettes of all things. But you have to have your account be approved to accept payment for "Mature Audience Content". That reads as "Any adult content delivered digitally including video on demand (VOD) and web-cam activities. PayPal may be restricted from processing payments in certain jurisdictions for adult DVD’s, magazines and other adult themed products or services."
But I am not an expert in these things, so do not take my word for it. But I would like to mention that if I were to "buy" your game, I would not pay you. I would pay itch. If I were to pay you, I would see your financial contact info. So maybe that makes a difference. I pay itch for access to the stuff you provided. And just because I did that, does not mean, I would be entitled to, say, a Steam key of your game or could make you provide me with a private download link, should I lose my itch account.
Regardless, if Steam can sell adult stuff, there has to be a way. People like money. How does Steam do it? They accept about any payment method you can think of, but I do not know, how they pay out to the developer.