Cheating at its core is a matter of breaking boundaries and lacking respect for the partner's wishes. People who do these things would more often than not be equally bad in a polyamorous or friends-with-benefits relationship because their lack of respect for other people and their limits is still there.
So in a polyamorous relationship for example, the person would break agreed upon boundaries in regards to how many partners the people involved may be willing to introduce, the rate at which new partners are welcomed, they may engage in romantic relationships with people outside the agreed partnerhood without letting anyone know (if such a requirement is agreed upon, which it often is in long term poly relationships).
In a friends-with-benefits situation the person might start sleeping with the partner's sister/brother, their close friends, they might start pushing them to do sexual activities they are uncomfortable with...
Basically cheating at its core is not a unique concept, it's one of the many forms that abuse and disrespect within intimate relations can take. People who cheat because they are poly, or gay, or aromantic or whatever are extremely rare. People who are those things AND cheat most likely exist in the same percentage among cheaters as they do in the general population, but their sexual/romantic orientation is not their main reason for cheating. People cheat for the same reasons that they neglect, insult and manipulate. They are selfish, broken, abusive and pathetic people.