I don't really like to get into the "NTR Bad" discussion since honestly both sides are quite toxic.
But as far as i remember from Subverse doujinshi, isn't MC supposed to be some sort of massive Memelord who doesn't give two fucks about who the girls fuck as long as he gets to fuck them too?
Of course idk since i haven't played the game myself but that's the impression it gave me.
Also, you know... you can try to... not project yourself into MC, maybe?
Like, if it is into the girl's character to fuck other people(or things) then sure. I haven't played the game but i doubt they will suddenly dissapear and never come back and then send you a video saying "Hey you suck so i went to another guy! K, bye!". Or at least not without the player having control over it.
And even the Mantics are under the playe's control. So if you don't want the rapey alien to get with "your girl", then leave the rapey alien away from them. Except for Lily i guess but that's her thing so nothing you can do about it.