I don't really have a stake in the whole NTR/not-NTR discussion, but my understanding of it is if you as the MC have some sort of agency regarding both your own sexual encounters with the object of your desire and their encounters with other partners, then it isn't NTR. There's an element of, shall we say, 'denial of services' from the object to the MC and that MC does not derive any enjoyment from that other dynamic that's required for NTR.
So in this game, where we are MC and D is our object of desire:
* MC can have sex with D = good old fashioned incest sexy time!
* MC can have sex with D who can also have sex with someone else with or without MC present, but MC is okay with that and may even get off on it = swinging/cuckolding
* MC maybe or maybe not can have sex with D who can and does have sex with someone else with or without MC present, but MC is not okay with that and certainly does not get off on it and is perhaps even emotionally harmed by it = NTR
This all seems complicated for the sake of complication.