Apologies if you already know all of this, but from 2013 to 2017 the US Department of Justice was applying a lot of pressure to American financial institutions to restrict their dealings with "high risk" industries. I think there were like 30 industries targeted, but the most notable ones were firearms sales, payday lending, and . . . porn. Payment processors, banks, and credit card companies felt like they couldn't be associated with a lot of things like virtual incest porn, so in turn they told companies like Patreon to ban that kind of content or else they wouldn't allow Patreon to use their services anymore.
On paper all of this DOJ meddling ended in 2017, but I think American financial institutions are still more than a little gun shy about dealing with these industries.
All that to say, Patreon may not actually be bigoted towards fictional incest porn, they may just feel like it's not safe for them to be associated with it. At the end of the day, they're just a player in the game too.