Monogamy has always been the way for humans since the ancient times. This is apparent from the way kids are raised. However, it also varies from culture to culture, and not all cultures practice monogamy. For most cultures, monogamy was the more frequent type of relationship, where an individual would have only one significant other and no more.
It is apparent this is the evolutionary beneficial one as it takes many years and a lot of resources just to raise a child. However, despite this, cheating has been rife. It has always been common, and always will be. That is simply human nature.