Addressing your lion example... As good an idea as that may sound, a large population can't be sustained by that kind of behavior. Luthor Rex made a great point in this thread. He said that when polygamy becomes dominant, or rather the elimination of monogamy, war and rape occurs. That's why you only see this kind of behavior in small groups. In a large population it would be chaos.
Really the difference in our perspectives is you tend to think that society is artificial. Artificial, the exact opposite of natural. That everything is molded like clay by us humans. But I believe it is the other way around. Monogamy remained dominant because it was the best and only way to sustain a large population without everyone turning on each other.
One last thing, you may be an expert on economy like you mentioned, but what makes you an expert on nature? A lot of times on this forum, you get a bunch of people that think they know everything about nature or human nature just because they read a couple of pop psychology books. I used to be one of those people, and now I realized how wrong I was. We think we know everything but we really have only scratched the surface. Human nature is infinitely more complex because our mind is so complex. Unlike other animals, we have the ability to make very intelligent choices. Basing the whole idea of human mating just on the way of how a male produces sperm is a slap in the face of the human race, because it ignores what's in your head, what makes everyone unique and not just a sack of sperm or eggs. I'm no expert on human nature (really, no one is), but our natural intelligence lets us rise above primal activity and polygamy.