I apologize for dragging this out but i still fail to see any real problem with it. All you have to do is introduce the NPC to the player when starting a new game and have a default value like "landlady," "roommate," or whatever. Then let the player change it if they want to. The fact that they see the default value of "landlady" should prevent any confusion, but the dev could also add a short description if it's needed. You could also give the player some sample sentences so that the player can preview their chosen word in context. You don't have to do this for all the NPC's. For players who don't care, they can just accept the default value.
The main problem here really is the context. If you want a decent incest game there simply should be a difference in dialog, reactions, etc. between MC baging his landlady/friend or mom. To me there isn't a really good compromise to be made, one option will always "bad" and in the worst case a compromise will be made so none of them is good.
I haven't played Dandyboy adventures, but I remember playing "Light of my life" which uses exactly the same system (choice + some dialogs to check) and - even though it's a good game - the default option seemed just so horrible that I had to restart it.
Next to that, does a dev really want to release a game that's pretty badly written unless the player adds the right ingredients himself? How many players actually do that? It's important to remember that incest is just one fetish among many. Sure, if you want to write a dedicated incest game, fine, but in most cases that's not really the case, either.
I don't use Patreon so forgive my ignorance, but do they really have a specific policy like that? It's not like they can prove it's intended to get around a ban on incest, you can just argue it's for allowing more player immersion through customization. You can't be held responsible for what a player chooses to type in. If this policy exists though, then i guess my suggestion won't work.
A couple of years back they banned quite a few of the big games because of incest, like Summertime Saga, Big Brother, Dating my Daughter, etc. and back then they didn't accept any excuse to unban these games. No stepcest, no patches, no nothing. At least to DC, the dev of SummertimeSaga, they made clear that if the incest would be back in any kind or form, his account would be gone.
So it's basically just a grey zone. As long as you are small enough patreon just doesn't care to check beyond the obvious, but as soon as you put stuff in your game that's real incest or provide official patches, etc. it's technically bannable.
So yes, in the most basic case the player can type in what ever he wants, but in that case the dev can't write a really nice story, either. As soon as there's actual background code working to adapt anything further to suit whatever the player has typed in, it's technically too much.
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