Isn't cheating the same as NTR?
They often go together, but not quite. Cheating is just the act of cheating. Seems to get tagged regardless of who's doing it (MC, or LI or someone else). And the person who gets cheated on doesn't necessarily have to become aware of it, suffer any consequences from it nor do they even have to be present in the game for it to be cheating, for the tag to exist. Cheating requires an existing relationship of trust/assumption of sexual exclusivity so there can be a betrayal of that trust. A wife or husband or girlfriend or boyfriend being unfaithful is cheating. Swinging/wife-sharing isn't cheating. A random chick the MC has tried but failed to seduce deciding to get together with his rival isn't cheating, there's no relationship/trust/expectation of exclusivity there so no unfaithfulness/infidelity, even if the MC similarly feels pain/jealousy from it.
NTR/Netorare on the other hand tags something that is experienced by the MC, from the MC's pov (as should the netorase/NTS and netori/NTI tags if they existed here) as opposed to something experienced by other characters (otherwise every NTR labelled game would automatically also have to be labelled NTI). It's about the pain/loss of someone you care for being taken away by another. It doesn't necessarily require cheating. The pain/loss of realizing your beloved wife/girlfriend/crush is hooking up with another is NTR, but so is them getting raped. The most popular example of NTR is probably the eclipse scene in the Berserk anime where the MC is forced to watch his LI get gangraped (even goes
berserk hacking off his own arm to break free and losing an eye in his attempt to save her). The fear, shock, rage, pain & loss he feels is NTR.
The diametric opposite of NTR is NTS where the MC is primarily turned on by the thought/sight of his LI with others, wants it to happen.