Not always.
the whole point is your partner is fucking someone else, but you don't get to fuck them, most of the time that's consensual, which isn't cheating. Sure, you dress it up as cheating for the fantasy, but 99% of kink/fetish stuff is fantasy/roleplay anyway.
so yeah, different side of the "All sex between any people who aren't the MC is NTR" coin.
For the record:
Cheating is when your partner fuck someone without your consent, you don't get to watch, and if you do stumble across it, you are not turned on.
NTR is when your partner fucks someone else, usually with your consent, and if not, you're ok with it, you do not get to join in, only watch, or hear about it afterwards, with the objective being your humiliation. You are turned on.
It's really not difficult.