Oversimplification. IMO, there are other possible evolutionary explanations than simply "it goes against our nature."
I had developed my own theory. Attraction and romance are pursuance mechanisms that allow cohesion to occur.
Romance and attraction are necessary in environments with constant life threatening danger and threats of separation (hence romantic novels are based on a theme where two people are constantly being separated by external forces). Various cultures in the past recognized that you could prolong attraction by making meetings between husband and wife forbidden, which is what they did.
When you take away the old environment in which these mechanisms flourished and introduce a new environment that causes these mechanisms to become dormant, than ya, you are going to get polygamous behavior because the mechanism that is needed for cohesion to occur cannot function, since the environment supporting such a mechanism is no longer there.
In the end, attraction and romance are pursuance mechanisms. In order for the pursuance mechanism to fire, there must be something to pursue. that is in fact a cue for the pursuance mechanism to fire. Attraction and romance are cue dependent. While our previous environment provided natural cues, our current environment is completely absent of such cues. We have to make our own.
Polygamous behavior again occurs because the threat of separation and various pursuance cues exist with a new potential partner, which again subside when the relationship involves long term cohabitation. If anything, safe and predictable long-term cohabitation is against our nature. Danger, the threat that you might not see each other again, that you might lose each other, competitors, predators, all acted as cues which ignited and kept the flame burning continuously. I say this because IMO, logically polygamy becomes counter intuitive when you have helpless and dependent offspring as humans do. Polygamy makes more sense when offspring can mature and be produced quickly. In humans, there is a great investment in producing only one offspring at a time, which is helpless and dependent for a long period of time. Monogamy in such a circumstance, makes more sense when it comes to survivability. You want long lasting cohesion. Hence jealousy is also a mechanism, ensuring monogamy. We keep a watchful eye on our partners, making sure there is neither emotional or sexual infidelity.
If our ancestors had something like a divorce rate in the past (of course there was no divorce since institutionalized marriage did not yet exist, but let us assume), and it was as high as it was now, the chances of survival for the offspring would have been significantly low, IMO. Low enough to question whether or not the theory that we are essentially polygamous really holds.
