I guess that's a way to make yourself stand out for a first VN, because NTR (basically someone stealing another's lover) is usually either a guilty pleasure or repulsive to many people. Though you usually hear about it the other way around where the girl leaves the guy.
NTR implies you're having an affair with them while still on a relationship, this game implies the goal is completely breaking them up so you have your boytoy for yourself only.