People say that a lot but I haven't seen actual statistics, and sites that have established low chargeback rates face the same issues.

Also that's not a reason to ban certain genres/kinks, which is what's happening here.

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.

Which makes less sense when you consider Steam will refund you game if you dont want it.

The fact that these were specifically incest games makes me think a title was somehow involved in distributing CSAM, which is often why Visa/MC crack down on porn websites.

But it is possible that Visa sensibly and correctly said "anyone who makes or purchases such a game is a despicable scumbag, and we shouldn't assume the financial risk of dealing with them."