NTR is when a woman systematically cheats on a man, while the man becomes aware of it. I specifically wrote cheating because the MC will be able to cheat on his chosen one. Not the opposite.
It will not be a harem in the literal sense of the word.
At a certain point, the main character will approach the period of time when he will have the opportunity (depending on the player choice) to be in a relationship with all the girls at the same time without them knowing of MC infidelity. So the harem in this case is a very technical expression and just a fitting tag.
