I wrote this comment before the one above. I'm open to other points of views if you write them down. I can't tell you for certain whether my statement is an absolute fact, but there seems to be some truth in it that is worth thinking about. Are you aware of any thriving, long-lasting society that rejects monogamy?
Iceland has low rates of marriage, but is fairly conventional about monogamy. Iceland also has roughly as many residents as Tulsa, OK - a population just large enough to avoid inbreeding depression.