Sorry, I thought ntr means cheating, like whether it's male or female
NTR isn't completely cheating, NTR, depending on which kind you're talking about, the most common used here is when someone important to the main character, whether male or female, is screwing someone else and it makes the MC "feel" like he or she is being cheated on and they get the pain etc from it.
It could be the MC's mom, sister, childhood friend, wife/husband. If the MC feels hurt because whomever is screwing someone else, it's NTR. (the most commonly used version on this site, there's like 4 different kinds of NTR)