Despite the noise of the information age, at the end of the day, humans still want to pass on our genes. But while it takes men between three and thirty minutes to do so, it takes women nine months (plus nursing, etc., etc.). And so a man’s reproductive potential is bound by the number of mates he can find, while a woman’s reproductive success is bound by the time/energy it takes to successfully raise her children.
And so the sexes battle: Genetically, men want more mates and women want help. And historically, resources decide the winner.
In cultures that evolved in resource-rich areas, females didn’t need male help to raise their children. Thus they gave little preference to males who would stick around to share the burden. These males, unfettered by paternal investment, quickly went in search of additional mates, but supply and demand meant they needed to fight for them—males in polyamorous societies tend to be big and horny, as it were.
But in resource-poor environments, women needed the contribution of hunting men, and it was all a dude could do to put enough meat on the table to feed a single family. Thus monogamy and paternal investment.
Boys Have Cooties
Due to the evolutionary pressures of male competition and selection for better hunters, most male mammals are larger than their female counterparts. A bigger body makes a better lunch for discerning parasites, and thus most male mammals have higher rates of parasitic infection.
And finally, in societies in which men had access to resources while women did not, polygamy developed—women needed (or were forced to need) male resources in order to raise their children, and certain men could supply these in enough quantity to support many women.
This is of course a vast oversimplification of a fiendishly complicated and much-debated topic. For a proper discussion, check out The Mating Mind by Geoffrey Miller.