As Steve above says, it's not a problem if the men are busy killing each other, and that is true, polygamy can exist in warring societies for quite logical reasons. That eliminates large numbers of young males. It can also exist in societies where there is a large disparity in wealth and power within society and then large numbers of powerless and poor men can be safely ignored. That is not our society.
The other option if war doesn't balance the numbers enough is to steal or buy women from others. That implies a slave trade of sorts.
