It's not the first time this has happened. That first time set the precedent that the payment processors have a vast amount of power over the transactions that can occur on the internet. There wasn't a realistic way to push back on it and so they will continue to expand this for... whatever reason they are actually giving. IDK - I would have thought that legitimate adult content payments would be quite lucrative for these processors to handle, it's not like they're beholden to advertising like YouTube is and their insane content policies.

I mean, I cannot find a valid reasoning for it apart from the vague term "high risk" which explains nothing. This is the best I've found so far:

The adult industry is no stranger to regulation and stigma. But in recent years, payment processor censorship has emerged as a subtler, more insidious threat. Companies like Mastercard, Visa, and their underlying bank networks often issue sweeping mandates, particularly regarding “high-risk” content. These decisions typically happen behind closed doors, without public accountability or stakeholder input from the communities affected.

(bold emphasis mine)

To reduce perceived brand risk or avoid legal ambiguity, even when the content is legal.

TBH they are making themselves look pretty shitty as a brand by moving sex work and other adult content back to the darker deeper recesses where it becomes less accessible and harder to regulate properly in terms of safety and legality.