Not really? More like as social animals, we prefer success to failure. It doesn't really matter what you do to the protagonist, no matter what he loses, no matter how he suffers, so long as, in the end, he finds some way to matter, changing himself or his world into something a bit better than it was before. Well, we are also prone to empathy. Empathising with someone else's suffering doesn't feel good, so you at least want them to be remembered. In many versions of Robin Hood, the story stops with his death. But even without children, with without a country, the reader can accept that he is a giant that changed the world toward the direction he prefers, in a way that won't be forgotten. In NTR? The main character, once the other two stop being amused by his suffering, could be abducted by aliens or fall off a cliff. The end of his story is that he doesn't matter at all.
The Protagonist in NTR isn't the main character, but rather the guy stealing the girl, as the defining feature of a protagonist is that his decisions matter. But the protagonist in an NTR doesn't achieve happiness. He's basically an algae eater, swimming along the bottom, eating what comes into his mouth, and pooping it all out again. It's just hedonism, mixed with a generous portion of sadism. Getting this particular girl/guy is never going to make him happy. The getting the girl in these stories is just a McDonald's cheeseburger of relationships. It fills the stomach, for a while. In a few weeks, he'll swim on and eat something new.
Whereas, the only important aspect of the main character is that he loses his (potential) happiness. There is nothing to like about him or his situation. He is completely unimportant to what is going on, and just the author/artist insists on telling his story anyway. You basically have no expectation toward his happiness. Nor are you even supposed to care. If he ends up dying early working in a black firm without ever dating again, it's all the same to the reader. And it isn't a story that will be told anyway.
The girl (role) in all this doesn't even have agency, being a creature with all the virtue and emotional depth of a stray cat. Often she's got the same sadistic streak as the protagonist. Most of the time she doesn't put any thought into what she does. She's also a bottom feeder in most of the stories I've seen. In a sense, the two who get together deserve each other.. though it's not like the relationship will last. None of the characters in the story achieve anything more than a brief comfort. All 3 will be no better off than they were in a few days/weeks. The best case for the relationship is probably Al Bundy. So why would most people like it? At best, you could manage some schadenfreude. But the main character is usually not a bad guy.. so why would you take pleasure in him suffering? And if you do, why not just read a story from the perspective of the protagonist or the girl?
Basically, the entire genre is an oddity from the perspective of its storytelling. It would be more enjoyable told by anyone but the main character's perspective. Even a 3rd party's.
