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."
I think the government should be the one deciding what makes someone qualify as a despicable scumbag, not a private payment processor that is essentially acting as a utility provider. For the same reason, I also don't think an electric company should be allowed to shut cancel your building's electricity if they don't like your mismatched socks.
That's a pretty wild idea for what someone would be putting on steam as a visual novel. And why would they need to be pressured into removing horrible illegal content?
Or you think one person did that and it made the credit cards decide any story with incest would be the same? That would be ridiculous on their part.