Actually, there is a sense in which polygynous marriage has not been the historical norm - even where polygyny is permitted, multiple wives are generally reserved for a relatively few men who can afford them or qualify via formal rank. For eons and eons, most marriages have been monogamous, even though most societies haven't been

The Moral Animal: Why We Are, the Way We Are: The New Science of Evolutionary Psychology (ed. Vintage, 2010) - ISBN: 9780307772749