If you are an american, you can sign this petition from the ACLU. https://action.aclu.org/petition/mastercard-sex-work-work-end-your-unjust-policy
They are a pretty big group with some actual political power.
Also if you are american, you can contact your members of congress about how you do not like payment companies being able to discriminate against legal purchases. There was a recent bill proposed that would prevent companies from doing things like that, but I can't find it atm.
If you are an investor of a fund that has stock in credit card companies, contact them about how payment processors caving into groups like Collective Shout could lose you money. Be fairly aggressive in your language, like demanding compensation, but not harassing of course.
Finally, if you use Mastercard or Visa, contact them about how you are disappointed in that they discriminate against legal purchases on behalf of random groups. If you do this, do not be agressive, but try to demand an actual person to talk to on the phone, and request higher management.