The amount of polygamy in a society is almost perfectly related to the power-differential in said society. Roughly every society with huge power differentials has some dude at the top with a wife, and dozens to thousands of concubines. Sometimes it's legal (All early agricultural societies). Sometimes, it's just that the feudal lord has all the 16-25 year old girls in the area working in his castle, with a back staircase into their chambers, for no reason at all.
Forager tribes run 1 wife per man, with occasionally a very rare man having 2 wives...but the egalitarianism built in to the tribalists is super-strong, mostly as a evolved wife-distribution mechanism. Guys with too much power get killed. It's only agriculture, which allows wealth (and power) accumulation, which in turn allows (and in all known early incarnations, guarantees) huge polygamous systems.
You want monogamy? Enforce equality, as the progressives want. It's the closest thing we're gonna get. On the other hand...you want an explanation of the drive towards equality (which I've never before understood): In non-egalitarian systems guys who aren't at or near the top of the heap don't get any nookie, or kids. As in, the top 10% of the men have 70-90% of the kids. It's probably evolved in real deep.
Matt Ridley's The Red Queen is the best book I've read since Kurzban's bunny book in January. There are friends of mine who didn't nudge me hard enough 10 years ago to read this book who have an accounting to give. More complete review late today, if my wife doesn't have a baby.