Considering their volume, I find it hard to believe that Valve couldn't find another, more lenient payment processor with similar fees.
Adult content has a high chargeback rate and high fraud rates so payment processing for adult content has higher fees.
You need to be more specific. Conflating "adult content" with porn is both problematic is masks the real issue. A large majority of games Valve sells are adult content. But as you can imagine grand theft auto is not causing a lot of political backlash, despite the objectionable content.