Even worse, some people just do not like to see NPC's having sex without MC there, lesbian or otherwise. Whether it is NTR or not, somehow for some weird reason they immerse into the MC in a male protagonist game and want to play his story and not mainly the sex life of other NPC's.
In this case though we are talking about a MC that has been faithful to his girlfriend to a fault, who is being told suddenly out of the blue that she wants an open relationship for whatever reason and that if she finds someone better as him during that, that she is moving on. Yeah talking about equal rights ........ and a fair option to refuse with no consequences.
Now that he has 10 ladies around that the devs of the game want him to spend time with, has no bearing on the fact that that is what happened to MC in the story as was told.