Monogomy is an economic construct. Change the economy and you may change relationships....

When we passed on our genes every sibling got the same. When you pass on possesions that it is a zero sum game. When humans got posessions that matter and move on to the next generation, then it made sense to mate with the partner that will give your offsprings the most possesions. 

The fact that you pass on to your siblings possesions and not just your genes, means that you can't have too many children from too many partners. 

In a world of abundance where the economic value transfered across generations becomes knowledge and skills and not possesions, what is the better relationship structure?