A response to the conclusion in Libby Copeland's piece for Slate on the effects of polygamy and monogamous marriage: "Is Polygamy Really So Awful?":
Ms. Copeland concludes that "Christianity may have brought monogamy to Europe and many other places, but those cultures succeeded because monogamy happened to suit them. In other words, as far as social evolution is concerned, the best form of marriage for a given society isn’t really about what’s moral, but what works."
Libby Copeland's concluding statement (that monogamy is best for society because it works, not because it is moral) gets the causation backwards: Monogamy works because it is moral. Christ gave his disciples tough standards in marriage-- so tough his disciples' initial reaction was "In that case, it is better for man not to marry." Nevertheless, Christ prevailed and his disciples embraced this moral doctrine -- and gave it as a gift to Western Civilization, not because it works (though it does), but because Christ so instructed (and still does, though Christians today on marriage -- as in many times in the past on different aspects of Christ's teachings -- are very lax in obeying and following him).
There is a unity in Christian teaching, and its fruits come out repeatedly, in myriad ways: the good of women, of children, of the poor, and of the sick are just a few (and all its fruits are good for society-- though, again, this is not why they are done). Monogamous marriage is just another in a long list of gifts to the West, and to mankind at large. We took it so much for granted we never realized it, until it started to fade. But ultimately it is not marriage that is failing, but Christians. Marriage of its nature does not fail -- people flourish in marriage, when they live it. It works, but it takes moral effort. Christians would say it takes more at times: lots of grace and effort.