It may help to take a step back and consider our species. We've evolved to be semi-monogamous, which is probably the worst of all worlds. But it's obviously been a highly adaptive strategy otherwise it wouldn't have persisted for hundreds of thousands of years. So instead of thinking of relationships in Disney terms, or like a game (games are where one "cheats" and reality is far too complex for simplistic ideas like this) it may be helpful to think in terms of hardwired behaviors. Women are unfaithful, men are unfaithful - because it's a highly adaptive strategy. Yes, it sucks, and yes, it would be lovely if we were either fully promiscuous like Bonobos or fully monogamous like gorillas. But we're not, and pretending otherwise is foolish - like being angry that we don't have wings on our backs.