What Constitutes Cheating?

We don't like cheating because it hurts us, but is that only because we have been trained to expect monogamy? I also think people have different conceptions of what cheating is and that's important to set up first. For some, if their partner would have spent time with someone else in a non-sexual way, that could be deemed as cheating. Whereas for others, things like kissing or flirting might not be a big deal.