Interesting. I have to imagine that it is easy to be monogamous when your species has a limited number of traits that separate individuals in any meaningful way. Say what you will about humans, but we are more complex than other animals, and that leads to quite a bit of variation.  Add in the fact that humans live longer than most animals, multiply that by the number of contacts with other interesting individuals humans may have, further multiply that by the wild differences in resource acquisition, physical attractiveness found in humans, and the idea that a human ought to mate like a parakeet appears more than odd.....

But then I must ask: Why is there such a propensity for humans to be emotionally hurt by rejection? Yes, we can argue that this hurt is driven by man made expecations and man made moral injunctions, but the question remains: why are humans so easily able to be hurt, rejected, depressed, etc. when they lose a loved one?