Yeah and that is fine, I mean she has made many attempts to try and sleep with him and he brushed her off, but at the end of the day she still cares and has feelings for her husband and is still married. So no matter if she was only getting dicked down by the MC, the fact remains ( that is the key fact here ) that if she sleeps or does anything sexual with/to her husband it is in no way/shape/form NTR towards the MC, not even close and that is what people need to learn and understand, hell her sleeping with someone else that is not the MC would not even be that she is cheating on him. The only NTR happening between Nicole and MC is the fact that he is bagging her while she is married so he is doing the NTR, where as he is ( boringly and terribly written in my opinion ) NTR'd by his teacher because of Chloe, however when they break up, that is not even NTR anymore as the requirements have been met, He lost her to someone else and he has moved on.
Just because he ( the MC ) feels like he wants to own and have Nicole for himself ( like some people do on this forum ) does not make it that he is getting NTR'd by her, He has influence over her to some extend but he has no full claim on her. Unless she decides to fully drop her husband and she completely decides she is going to be his, there is no NTR on his side.
So while I get what you are saying still does not change that the MC is getting NTR'd by Nicole. People can hate it, dislike it, whatever they want. But just like Nicole said "He is my husband". At the end of the day, people are throwing the NTR word around that it has as much meaning as the word "privilege" now a days, overused and abused.