there is no ntr in the game, or are you talking about the harem? this is a harem game, designed to be played that way to see all the content. its going to be written in such a way as the girls will all be ok with it and be happy, thats what Cari does is harem style.
you can just take the decisions for one girl, but its going to be a pretty empty game, there will be lots of events you will miss, nothing changes with a girl in their relationship if you are with only them.