NTR only gets a pass in porn because it's porn. The story exists solely to jack off to the sex scenes. We don't need to like the characters or have the story make sense.

In mainstream media, a heroine even THINKING of cheating while in a relationship is enough to turn off a whole fanbase. At it's core, it just makes the heroine unlikable in the worst way. Regardless of what personality she had at first, 9/10, she turns into a vindictive, brain dead slut. A heroine could get away with killing an MC's family as a yandere more easily than she could kissing someone who's not the MC.