Well, cheating objectively is betrayal of trust. If you promise to be exclusive, then you brake your oath when you are non exclusive so this is kind of obvious.

But what i am curious about is what you people think about the expectation of exclusivity in the first place? It's easy to say "you broke my trust" when you made him/her swear to be loyal or else you wouldn't be with him/her. Many people do cheat at some point in their life, it's almost 50% of both genders in some countries (like Thailand), 1 in 3-5 in the US. Yet, it's far more then 50% of the population, in fact the vast majority of people who do expect loyalty. It is very likely that there are plenty of people who do expect their partner to be exclusive, but do not hold the same standards for themselves.

Given the above, it seems to be clear that human beings are not 100% monogamous by nature, both monogamy and the practice of multiple sexual partners were prevalent troughout human history. So why do we expect loyalty as a default condition? Why we expect each other to make false promises? Is this not possessive? Do we not trait people as property? What is objectively wrong with having multiple sexual partners, while someone is in a long term relationship?