Well to be fair, incest is more taboo whereas getting cucked is more like a slap-in-the-face. I mean seriously do you really see roleplaying as family members in bed with your girlfriend on the same level as watching your girlfriend get fucked by some other guy??? Think about what you're saying here. Of course that was just analogy, but I'll put it into a more literal perspective.
There's a HUGE difference between enjoying participating in something considered "kinky" or "naughty" by a lot of people, versus standing by as you watch someone you love cheat on you (it's essentially a big "fuck you" to the player). This is especially true given that in one situation YOU THE PLAYER are involved, whereas the in the other one you aren't. Honestly, how much would you enjoy the game if you played as Morty, but ALL of the sex scenes were for Jerry and the girls and you THE PLAYER only got watch? Now imagine you DID get to have sex with say Beth for example, and throughout the story you're heavily enjoying progressing your relationship with her, but then OUT OF NOWHERE Jerry comes by and fucks her instead of you getting to fuck her. How would you feel? It's a DOUBLE "fuck you" to the player.
Now you may find this situation enjoyable, which is fine (I would think you're fucking disgusting if you do btw, but I digress), but hopefully this explains the perspective of us "NTR Haters". At the very least it does for me on why I absolutely HATE NTR.