It's not about being taboo as if it is morally wrong, it's just that most people don't like it, but unlike something like foot fetishes or something that you can just ignore if it happens to you in the game, when it happens and you don't want it, it sucks and can ruin the game for people. For example imagine you're playing a game and you're straight and there's nothing that says the game has gay content, then you have an unskippable scene where some big dude comes and fucks your male character in the ass. You probably wouldn't like it and it would kinda ruin the game for you. It's the same for NTR, having someone come and steal your girl.
The issue is that historically a lot of games with NTR did not mention it was NTR. my guess was that those were japanese games and they didn't really give a fuck about advertising it properly if that happened and you didn't like it, so when it did, people would hate it if it was not advertised. and this hate continues to this day. Personally, if it's properly advertised that a game has NTR, I think it's fine for most people, they will either just ignore the game and not play it (like me) or they might bitch about it either for the meme, or because there is some hate towards NTR they think it's acceptable to do. so the conclusion is, you pretty much keep seeing hate against NTR and the cycle continues