Monogamy comes from the time when marriage made wives their husband's property

Very wedding ceremony reveals the nature of marriage: father 'gives' his daughter (his property) to another man. This giving of property to another man is a financial exchange. Originally and up until very recently, love wasn't part of marriage as can be seen in medieval times when nobles traded their daughters to other nobles for financial or other tangible gains. And while we've recently made it about marriage and romantic commitment, the actual institution has nothing to do with this and they're added on top.

Only about 5% of mammals exhibit monogamous practices and humans aren't one of them. That doesn't mean 'cheating' is ok however. Cheating implies a breaking of an implied or explicit statement of monogamy. It gets used since we (in the US) are a defacto Christian culture where monogamous relationships are the implied assumption. So any and every relationship begins with monogamy the implied model.

During the sexual revolution, experimental alternative models were tried with varying success. But the monogamous paradigm never changed and it remains. I think this is less about it works or is superior to non-monogamous models, as the religion monogamy comes from is still the majority. Until Christianity is relegated to the dustbin of history, the paradigm of monogamy will likely to continue. Though there's signs this may be in flux, change comes slowly, never as slowly as where religions are concerned.