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Powerless MC being completely dominated by the Netori is only one, usually very extreme form of NTR. NTR is jealousy and betrayal. You can be in a completely platonic relationship with your best male childhood friend, but the moment they start to live a better life than you by gay fucking the guy who beats your ass for lunch money, and not using that relationship to protect you, you're the victim of NTR. Your little sister, who you also have a normal family relationship with and not an incestuous one, could start having sex with your boss, and he's a complete dick, and her relationship with your boss doesn't make your boss any better, she knows full well that he's a dick to you, she even allows it or enables it. Boom NTR.
So no, NTR doesn't require you to be powerlessly watching something happen to your lover. It means that someone has betrayed you beyond return (more than cheating as you said) and is about the feelings associated with that betrayal, usually jealousy, but not necessarily.
Similarly, the netori, doesn't have to be 1 person, or even a person at all. Say the MC is a 12 year old boy. His single mother gets addicted to drugs. As a result of her drug addiction her relationship, which used to be very positive, changes, she's no longer the caretaker that she once was, she has betrayed you. No sex required, the drugs themselves and her addiction to them are NTR.
NTR is a relatively loose concept in this way. While many instances will be the heightened forced cuckoldry that you're describing, they don't have to be.
Netorase is also abbreviated as NTR, and Netorase is a scenario where the A goes and finds a Netori, C, to come and fuck their lover B. Or A creates a situation where a C and their B will come into contact. Or A finds out that there already is a C and decides to stick around. What I'm saying is, both male and female masochists might get off on being cucked. It's fine if you aren't one of them. I'm certainly not, as a sadist, but that doesn't mean that these people don't exist.
In the end, your experience is not universal. Things that you might hate, other people might like. This is something you should always consider.