Never, I hope, if MC is not involved by instigating it. A game it´s not a movie or a novel. It's not even a Visual Novel. The player is not just a viewer. He takes decisions, so he must be involved mostly in everything that happens around him. The player is the MC, and it's called "Main Character" for something, after all. If not, where is the point? If the other characters are doing important things on their own in the house that MC (that means, the player) don't know (having sex, romances or whatever), you only make the MC a SC (Secondary Character AKA "doormat") in his own game, and also, look like an idiot that doesn't even know what is happening in his own house.
And, over all, be careful. Any form of NTR (lesbian or not), is a huge NO-NO for a lot of people, even if it's just hinted. If your idea is that there are no romantic feelings in this game and MC and the girls just like to have sex because of Rae's aura or whatever, and after leaving the house they are going to mostly forget about each others, you should make it 100% CLEAR.
But it seems that some characters are developing feelings for MC, being cute with him, cuddling with him, kissing him, actually caring for him... And MC (the player) could get fond of those characters. It's a role game after all, the point is the player forgets for a second it's just a fantasy and gets involved in the story and the characters... If it happens that the MC (that means, the player) for example, enters a room and sees the character he is fond of, having intimacy with another character, even a girl... you are going to kick a huge hornet's nets. A lot of games have gone shout for that.
If the main theme of a game is not NTR, jealousy, rejection and cheating and aren't aiming specifically to the fans of that kind of fetishes, intelligent devs don't go even 1000 miles close of those themes. Games like this one are not real life. Are fantasies, and most people don't like to feel any form of jealousy or rejection in their own fantasy. It's just a friendly warning.