Really, NTR by itself isn't the problem. The definition is too widespread and there are too many different angles for anyone to be talking about the same thing. Leading to just a bunch of people being upset nearly all the time.
NTR can, to some people, be the heroine having sex with people other than the protagonist. It can be cuckoldry. It can be cheating. It can be the protagonist stealing the heroine from her significant other. It could be mindless (MC), or cruel (humiliation), or encouraged (apparently) by the protag. Note: I am talking about 'perceived' definition here, not actual, so put your dictionaries away. We live in a weird timeline where people are always trying to alter definitions for their personal benefit, so you need to target perception rather than "terms" to find a solution.
Really, while some people may disagree, I find the worst offenses that cause people to revolt lie in the intention behind it. Primarily one of two things would need to occur before I find it completely distasteful and ignore the game:
1) Should the heroine have sex beyond the protagonist, she shows a preference for that sex, instead of for the protagonist's.
2) Should the protagonist have a romantic interest in the heroine, and she for him, and she engages in sexual activity without his consent.
In both these situations, if it were real life (I use the term loosely), it would simply be a wake-up call for the protag to simply dump the heroine and move on. Game over. Both situations exist only to harm the protagonist, and its what I dislike about the entire fetish. (And its not even play-humiliation at that point, which behind closed doors can even be fun, through teasing and the like. But rather the beginning or complete aversion to the protagonist.) A threesome is not cuckolding, but if you and your friend are doing a girl that you want to be emotionally linked to the protag, there should be some sign that she prefers the protag in that encounter, even if not for superficial reasons ("bigger dick").
Now for an extreme example to illustrate my point. You have a male and female. The female locks the guy in chastity. (queue first screams of cuckoldry) The female also brings over a guy, with the first guy's consent. (more screaming) And the female pegs guy #1 while sucking off guy #2. Could this be in violation of the rules? Yep! However, should the female express that she actually was more sexually satisfied fucking guy #1 in the ass, rather than getting fucked by guy #2, it stops violating the rules. She stops seeing guy #2, and instead takes her pleasure from domming guy #1.
Now if she prefers guy #2 for sex, and just stays with guy #1 for whatever reason. This violates the two rules and creates the aversion.
Superpowered does skirt that line I drew. Mainly in the concept of Nymphomania (Last played 0.12, so things may have changed.) When someone you trigger nymphomania in has sex in the bathroom, you start to encroach upon #2 (without consent). The question is if you mindbreak someone to trigger uncontrollable sexual urges, are you consenting to them having sex with others. It could easily be argued both ways, imo.