ok, first I want to say that I understand both points of view, it is certainly annoying to have a character like darian around, but I consider that there is much more satisfaction in fucking his girlfriend while he is around without knowing that it happens and little by little make her become more loyal towards us and more indifferent towards him, until it is her decision to kick him out of the house (if that at some point happens, which I also doubt).
But throwing him out of the house, takes away the whole point of the protagonist being the one who does the NTR, since it wouldn't matter anymore.
I also know that the NTR will always be a problem cuz no matter if it is netori or netorare, there will always be people from the opposite side to complain, so I just wanted to offer my point of view, anyone is free to have a different opinion.