We do have a sort of polygamy but the fathers don't stick around! But it's not hard to see why so many cultures had (and still have) this type of arrangement where wealthy men that can support all these women and kids have many wives, average guys have 1, and the bottom of society has none as they can't support them anyways.
Poor men can have multiple girlfriends and one night stands as well. The vast majority of men thrown in jail for failure to pay child support have low paying jobs.
