Steven Pinker, in The Better Angels of Our Nature, about which I blogged recently, argues that the civilizing process of the modern era is one of the main reasons for the massive decrease in violence in the world.
Pinker does not treat the change in marriage norms specifically in his consideration of the civilizing process. I have long thought, though, that one of the great social benefits of monogamy is that it should reduce the violence by groups of unmarried men that is a predictable by-product of polygamy. This, I think, is why monogamy is nearly universal in developed nations, despite the wide variety of religious and cultural traditions that these different nations developed from.
A new study by Joseph Henrich and others at the University of British Columbia found "significantly higher levels of rape, kidnapping, murder, assault, robbery and fraud in polygymous cultures found in Asia and Africa." By contrast, monogamy leads to more paternal investment, long-term planning, economic productivity, savings, and child investments.
These social advantages explain why even those who benefit the most from polygamy - rich, high-status men - are willing to switch to a monogamous social norm. The civilizing process leads to monogamy because of its greater peace and social investment for society as a whole.