Edit: I absolutely fucking abhor ntr, there's probably no one out there who hates it more than I do. I had trouble sleeping that night the first time I encountered ntr I'm not kidding. It was a hentai. So believe me when I say that first scene with the ex was not ntr, least not for me, somehow, idk why. Sure, it sucks cause bitch wasn't even cheating with a better looking dude with a bigger dick, but with a woman with a fake plastic one. That shit is incredibly insulting and emasculating. But Idk, somehow I don't think that's ntr. Although for the benefit of new players maybe make that scene optional to see?
As someone who doesn't visualize themselves as the MC in any content format, I enjoy NTR (seeing a girl enjoy having sex is always a good time) and have definitely consumed a lot of NTR content in the past.
So that being said, I also
personally don't think the opening scene is NTR, because at least for me, NTR and cheating have one key difference: NTR
has to have some emotional connection, whereas cheating
can but isn't necessary. The game starts almost immediately with the fiancée having sex with someone else. There is no question that this is cheating, as the MC clearly isn't into this (though judging by his character in-game, if she had talked to him about it beforehand he might have been, oh well). But we don't have any emotional attachment, backstory, or anything of the sort, because it's the very beginning of the game - all we know is that they're obviously together, but the first we see of her character is that she's getting railed by her boss. Perhaps this is just my limitation from not seeing myself as the MC, but I would think that people who hate
cheating would hate this.
NTR, though? In all experiences with NTR content that I've had, there is
always a relationship that is built up between two characters
before the NTR content starts to take place. For me, this isn't NTR. NTR is when you start the game with a loving kiss and nice banter with the fiancée
before you discover her cheating. Or when you establish that the girl is your childhood friend and you have feelings for her and she clearly has feelings for you too. Or any number of other scenarios.
So in conclusion, I'm pretty sure that this game is slightly mislabeled, and people misuse the NTR label pretty often (or perhaps I myself don't understand it perfectly). But NTR is
always cheating, whereas cheating isn't always NTR.
Also, I'm not sure that making the scene optional is really feasible - it's pretty much the driving force of the entire game, and it's pretty important to understand why the MC is doing anything thereafter; his actions don't make narrative sense otherwise. I guess you could ask the question "do you want to view the cheating scene?" and then if someone chooses no, just make a very short typed description of what happens, though.