I've spent a great deal of my adult life studying these issues. ChrisBennet has it exactly right. Monogamous species are in fact the exception rather than the rule, and monogamy is only practiced when it benefits surviveability of the species.
Is monogamy "natural" for humans? Of course not. We evolved into this efficient, aggressive, dominant species because alpha males have enjoyed greater reproductive success than their more timid counterparts.
Is monogamy relevant to modern life? Absolutely! In order to live in an organized society we must learn to subjugate and repress many primitive instincts that were critical to our survival just a few hundred thousand years ago. Hyper aggression and a fast-twitch violence response might have kept your cave full of meat and intruders at bay, but the same responses during rush hour in Los Angeles will get you arrested.
Just because the genesis of the urge is "natural" does not imply that it is appropriate or excusable. In many ways, the ability to control the darker angels of our instincts is critical to our success.
Humans are intelligent, inventive, sentimental, curious, lustful, violent and dangerous. At some point, all of these traits have been critical to our survival.
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