Unequal gender ration in marriages = certain population of one gender left in the dust. I'm pretty sure this has been true with every polygamous society, plus it's just common sense.
These are from my earlier post:
About 78% of human societies are polygynous, in which some men marry more than one wife.
-Murdock, George Peter. Ethnographic Atlas.
Only 22% of societies are strictly monogamous. No modern societies are polyandrous, in which one woman marries several husbands (not counting extramarital sex, and a poor region of India and Tibet where women marry brothers because the work of several men is needed to provide resources to raise a family). Only 3% of mammal species in general are monogamous, although at least 15% of primate species are.
-Insel, T.R., Winslow, J.T., Wang, X., Young, L.J. "Oxytocin, Vasopressin, and the Neuroendocrine Basis of Pair Bond Formation," in Vasopressin and Oxytocin: Molecular, Cellular, and Clinical Advances, (Plenum, 1998, ISBN 0-306-45928-0), p. 217.
...And I don't. But I'm curious, what makes you think that we will?
The ever increasing trend of older people moving back in with their elderly parents, increases in extended families living together, the increase in divorce and repeat divorce rates, increase in publica acceptance of single parents. In general society is shifting as a whole in terms of relationships. We're beginning to open up to new ideas instead of the same old stereotypes and norms.