Will they want to see that? As I said, I like some NTR scenarios, but I don't care about family stuff and this very specific situation wouldn't arouse me. Tags are not for very specific scenarios, that's why we combine them. It's like having a tag for "games where the MC's sister enjoys anal but doesn't let him cum on her face". A better tag for your situation would probably be something like Humiliation.
Yes, it's an established trope in NTR and is quite popular. Goblin gave a very western example. This type of cuck BBC humiliation stuff is pretty common in the western scene. You could argue that these could be tagged with BBC and humiliation to give a pretty good idea on what it's about.
There are more japanese tropes where the NTR isn't really about the humiliation (at least until the very end), and more about the loss of the caring and peaceful relationship between MC and his mom/sis/childhood friend. He will slowly lose his bond with these family members. Maybe you don't value family bonds very highly due to cultural or whatever reasons, but there is a real essence of betrayal when a mother, who's first priority in life used to be her son, starts to turn into a whore who only cares about getting dicked because she got curropted by some chad/old bastard. The western society which values freedom and individuality might see this scenario and say "So what? She is free to fuck whoever she wants. You (MC) are a grown ass man who should move out and live his life."
Eastern societies typically place great importance on the family unit. There are expectations of reciprocal care. Parents rear the children and care for their well being, and as they get old, the children in turn care for them. No putting them in nursing homes and shit (although this is changing due to westernization of most eastern societies). You are to respect your elders, and they should act in a manner to deserve that respect.
As for the childhood friend thing, that is something that could baffle westerners as well. "Who gives a fuck about a childhood friend, lol?" True lasting friendships are almost as important to one's life as family bonds. This is more true in regards to male-male platonic friendships. The best we can do in English to denote close friendships is BF or BFF. This term is used so loosely, and the strength of friendship so flimsy, that you could change BFFs like changing clothes. In Korea, there is a term called "balls" buddies. It's basically friends that grew up together and pretty much are as close as family.
Now why did I go through all this explanation? When a mother who should be worthy of respect and care about her son's well being starts to lose her shit and abandon all motherly duties, that is perhaps an even stronger form as betrayal than just your GF fucking some other dude. Same deal with a childhood friend who should always have your back, regardless of romantic feelings (if male and female), turning on you and throwing away all those years of built up friendship is quite a blow. That is why it's NTR and the different cultural expectations makes it hard to find an equivalent word in English to describe such a scenario.
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But I do agree that 99 out of a 100 F95zoners simply don't understand the term and have been using it in an increasingly absurd manner. Basically wanting to use it as a shield against content that could harm their ego, rather than as a tag that people could search for to find actual NTR focused stories.
As you say, there are actually a great deal of info to be had with the available English tags like humiliation, sharing, cheating, without having to co-opt a specific tag and misuse it to confuse people both for, and against, the content.
