US civics 101. The first amendment mostly restricts government action. This is not a free speech issue unless you want to legislate that adult content is a protected class or want to make a special clause for payment processing.
This is a perfect use case for crypto imo.
If you are making an argument that new legislation needs to made, great but unfortunately people jump to the idea that this is immediately a free speech issue.
Could a case be made not from a free speech POV but from a antitrust one?
Freedom of speech is not defined by the US constitution. Free speech is an ideological stance, not a legal definition. US laws protects some forms of free speech and not others.