The winner takes all for the right to reproduce.

You really meant to write that, didn't you?

LOL,

In an undergraduate class we were given a study that showed that the average heterosexual man reports having 9 sexual partners through their thirties (I don't remember the exact numbers) whereas the average woman reports having a lower number (let's say 3) sexual partners.

The professor wouldn't address the mathematical impossibility that these figures where correct even when I asked him to.

I suspect reporting error.

The book is focused on pre-historic society. As someone mentioned earlier, our current social constructs are strongly influenced by money. In a hunter, gatherer society, different rules would apply. If children were really raised communally and there were no questions about inheritance (or even understanding of the mechanism through which conception occurs), there would be no need for monogamous relationships.

Not sure this is true. I think human females still go for human males with sixpack abs and big big bicepses, even though they're no use in modern times.

There is the pleasure principle at work. Abs means ability for sustained rhythmic poking and big biceps means no interruptions during poking sessions. Really simple!