Ah but one of the paths you do have a relationship with Rose, you can have a threesome with her and Chole, with the MC expressing an interest in doing it again, or even furthering a relationship with rose. with myself and many others here that are complaining, once you had sex with her, you entered a form of relationship with her. Thus NTR when someone else takes her.
In real life, yes someone you just had sex with, is free to go and have sex with others. But this is not real life it's a game, a game where the player, is showing interest in a particular group of characters. many people that, don't like having the character they are playing share the girls they peruse, have no issue with doing things with girls that are already in a relationship. It is perceived differently. Perusing someone already in a relationship, allows for the knowledge that they will be having sex with their significant other as well as you. Perusing a girl who is not in relationship, lends the player to feel a sense that they are exclusive to the player, that perception of intent to share is not present. Unless the game expresses that there is NTR in the game.
Games like The Tyrant have NTR listed even though the MC is not in any exclusive relationship with any of the girls in the game. Why because he is still perusing them sexually and so are other men, to include their significant others.