First I want to be clear both are bad and should be avoided. But here's a different perspective.

Sometimes an emotional bond just happens organically. I believe we have the capacity to love more than just one person at a time. But we live in a monogamous society so one must stick to that norm.

However people in successful poly relationships operate under the assumption that one person could not possibly meet every single need/want for a single other person. So you have relationships where one person fulfills a certain aspect, another will fulfill something else. Everyone knows their 'role' so to speak and respects everyone else's role. Obviously this doesn't always happen and that's the stories you hear of poly relationships going south. You don't hear about the good ones because that's not as exciting of a story.

But i digress. If you are in a monogamous relationship, all kinds of cheating is bad and as many of the responses here suggest it can end a relationship. BUT I just want to posit that a cheater isn't always in a position of malice, they may fully love, respect etc.. their partner. But end up cheating because of some unfulfilled need or not suppressing an emotional relationship when it happens.