I don't think it's correct to apply the "are meant" term here. There is no specific purpose in nature, other than to stay alive and reproduce. Is it efficient? No, but neither is homosexuality, and BAM! Gay penguins. To be "meant" by nature doesn't mean it's "correct" on a superior moral level.
On the other hand, humans are expected to be monogamous. It's in our genes, we are possessive by nature, it's what has guided our evolution to the top of the foodchain. We are ambitious and possessive, pretty much like most of the animals. So a person usually fancies to be identified as the best and monogamy is a pretty fast way to do so.
I don't think polygamy is "better" than monogamy. More efficient for reproduction, worse according to our instinct, and something that directly clashes with the boundaries of the different societies in this planet.
What I do can say is that I'm monogamous. I can't think of sharing someone and I follow other principles (such as deciding it's more important to keep a friendship) well before considering the reproduction factor, so I prefer conceding exclusivities as opposed to more varied poontang.