Our ancestors (the evolutionary one, not social one) were primates. Primates, even today indulge in polygamy. Hence its not surprising if I told you polygamy is practised in some human cultures in one form or the other. So this means polygamy was a good part of our evolution.
Then why is it not as popular today like it was before...?
Right now I believe monogamy is more of a social construct rather than pure natural evolution. In other words I say it is social evolution. It kind of forms the basis for humans to be able to live in a society. There could have been a time, in the nomadic or in the agrarian days, where the males actually decided to invest in reproduction.
Investing is good thing for the offspring. There is more guarantee that the offspring can reach adulthood; by then he or she would have also learned necessary skills to go about life. The group of people had an idea of the skills they needed to impart to the next generation. So on one part it was about training the generation.
There was another socio-evolutionary aspect of this kind of investment. It was that the group needed more of these trained generation to advance their society. Indirectly, we can conclude that the law of survival is still obeyed. And so is the gene expansion program being obeyed in a restricted sense. These two factors could have led to the group's survival.
The males didn't have to fight wars over trying to possess a female anymore. This meant fewer injuries. Plus, their investment into parenting, training the child, etc play an important process in social evolution.
From the reasons I've stated here I believe monogamy is about being able to live in a society. Monogamy gets all the advantage of a nuclear family too. But one should not think that monogamy was the definite reason why the society is a lot stable today than erstwhile. In fact we have no proof that monogamy really leads to a stable peaceful society. But considering the graph so far I think this is what it appears to “be”. But don't be deceived.
But we still see violence happening in society due to many reasons. Tensions of war, crime, etc all plague society. Monogamy has done its part, and is probably still doing it. These problems are rather lack of resources to counter the rising human population. Imagine those little fights we had over a territory or a person, or on anything you are very much dependent on. The only difference is that they are now on mass scale.
Update (12th Sep 2013) : added some bundles about monogamy - http://bitly.com/bundles/deostroll/w