I really hate the idea of not having access to all the girls, especially when every other character has agreed to share. Asami even agrees to share with our own stepmother, so why would she be offended with her mother? Arguing that you need to keep the game grounded in reality is a bit far-fetched, especially given the already existing story.
I do think it's certainly fair to make the characters act as themselves but a "fuck reality, common sense shouldn't exist, I want to fuck everyone" button would be a fun idea in the future. Just make all the girls available with a toggle and warning that it would kinda make the characters "break character" if you will. Either way, good game but adding in that kind of cheat would please anyone who could have an issue with characters seeming like real people and not cardboard cut-outs for porn purposes.