I actually like stealing other guys' wives or going after other characters' partners in games, so I don't see a problem with it when I'm the one doing it. But having my girlfriend or wife getting fucked behind closed doors in a game? I hate that shit. Back when I watched hentai, there was a phase where literally every single release was NTR, and it was absolute hell. They always start with a happy, wholesome couple and then some ugly bastard shows up and ruins everything. It's just so heartbreaking.
I think it could still be a thing even for games now. I remember several times, when I was searching for new games, situation like this:
Me: "Oh, this game looks interesting, I should open description"
Description: "In this NTR game you'll..."
Me: "Oh fuck, not another one..."
Bonus points for cases in a row.
As for hentai - maybe it's only my thing, but I still find it a bit less unbearable than games, since, imo, it usually is not so centered on some MC. And I can't associate myself with them (though I'm not doing it in games either. But still - when they give you a character you can control - it still somehow connects you with character).
And I think I feel kinda similar for Netori stuff - especially when game allows you to choose how do you want to proceed (since I'm trying to be nice, if game allows it). If it not forces you to really play in bastard-style.
For the whole NTR thing from thread question - I don't have a sharing fetish (if it's about literal cuckoldry) and I don't want to play as a herbivore, who can't fight for their partner. That's why it's really annoying to see this kind of thing in story. I'm more or less okay if developer gives you a way to avoid such scenario (it's absolutely logical - you can fight for partner and you can win. Well, maybe

). But when it comes to unavoidable NTR - it's like you're starting a game while being a loser by default. What's the point?
"Sorry, Cuck-kun, I know you're trying hard, but this Ojisan is simply better than you in every way - he's fatter, balder and uglier than you. This is everything I could dream of!" - I just can't take that seriously. Maybe I'm thinking too hard about it though...