I see a lot of valid points here and will probably put something like that in game sooner or later. Maybe not the fact that MC just watches, I always want him to participate. But that the girls maybe say "We started a bit without you, but now that you are here..." Which is only fair, it shows that the girls can love each other as well even though there is the taboo aspect of the roomate/landlady situation etc.
Now I just have to find the right girls and the right moment for an event like this. I already have the next
threesome planned that will be in 0.11. It's one that people will probably not expect but will say "Holy shit yes..." Especially because I will put some very lewd dialogue in there. ( Not, it will not be between Lucy and Alice you pelicans
). It will probably be the best threesome in the game so far. <- That is a 0.11 spoiler btw.