NTR is not literally what the word means, its an entire genre, and the genre is a lot more flavorful than "just cheating". It can involve significant people that arent SO, like sisters or mums, its more about the taste of someone taking away from you someone important for you, thats why "just cheating" its quite lame in comparison.
Also OP is right, Polly is just cheating and its kind of the most boring of all the girls.