Where did you read that? Humans evolved to be monogamous, and monogamy is the most common form of marriage in both contemporary and historical societies. Many of these societies also allow polygyny, but its practise is less common; usually only high status men with a lot of property can afford to have multiple wives. Since accumulating property is a relatively recent phenomenon (last 10,000 years or so), polygyny probably is too.

Monogamy is an ambiguous term, though. It would be more precise to say that humans tend to practise serial and social monogamy. Serial meaning that while people might have more than one partner, they only have one at a time. Social meaning that they only have one socially accepted partner but can be sexually promiscuous at the same time (i.e. we cheat). So monogamy and promiscuity aren't mutually exclusive. That's probably as true today as it's always been.