Even if we waive homophobia as a possible motivation entirely for a second, it still comes off as, I have to say, really fucking pathetic to be so dead-set against Maria having sexual encounters with other girls you're dating that it legitimately harms your experience of the game and you can't even stand to look at it, as the original poster expressed.
The idea that it's "like NTR" if two girls who are both getting regular dick from you and may even have been in threesomes with you already might sometimes fuck each other when you aren't around is so sad I can't help but laugh. How fucking shot is your self-esteem that you find it threatening to your manhood or whatever that the horny bisexual girl who has gone out of her way before to help you get your dick wet maybe wants to get in there, too?
And moreover, wow is it some really fucking dehumanizing shit to say that none of these girls you're fucking are allowed to have any intimacy with each other without your express fucking permission. I get that they're called your "harem," but maybe don't base your expectations for how that kind of relationship should work in this character-driven porn game on the behaviors of Chinese emperors and cult leaders. Only you are allowed to have sex not just outside of a relationship, but even with other people within the relationship? Maria is flat-out not allowed to have any sexual partner other than yourself, male or female, ever, while you're fucking random elf girl strays you picked up over in the shelter you built for them that she hasn't even met yet? That's some next-level Incel shit, I have to say.
It's one thing to just not be into girl/girl (I'm actually not super into it, myself, though I can still enjoy it), but if your reaction - to fictional characters in a porn game, it must be said - is so strong and vehement that it makes you close your eyes when Maria has sex with one of the many other girls your self-insert MC is regularly fucking because that legitimately feels "like NTR" to you, maybe you're the one with the problem.
So, you know, maybe do some soul-searching on exactly why you feel that way. Get some counseling, if you can. Or just go to 4chan and you'll find plenty of like-minded spergs to agree with you 100% and you can scream about "cuckshit" together.