Patreon is in the business of making money. If they decide not to allow incest it is almost certainly because they believe it will get in the way of them making money. The two most likely reasons for this are 1) They are afraid of possible legal action, and 2) They think they will lose advertisers that won't do business with sites that have incest content.And I'm not saying every porn game is high art, but it's a creative venture and Patreon has no ethical or moral right to enforce censorship.
Pretty sure that ethics, morals and censorship don't have anything to do with it.