Here's the deal. It's all about how much time and effort males invest in the next generation.
Peacocks copulate and cop out. The peahen goes off to incubate her eggs and raise her offspring. So there are always more sexually active cocks than hens, which means greater competition between males, which favors the males who are best at fighting and looking good. And you know the outcome: a huge degree of sexual dimorphism. All that male strutting and aggression and showing off. What a tale! What a tail!
By contrast, male magpies invest lots of energy in reproduction. They copulate, of course. But they also feed the incubating females and provide most of the food for the hatchlings. These responsibilities limit the male magpie's opportunities for hopping other hens. The result: Similar numbers of sexually active males and females, not so much male competition, and very little dimorphism. Male and female magpies look pretty much the same.
The name of the game, of course, is to get one's genes into the next generation. Two strategies: both seem to work for their particular species.
So what about male humans? Do we have in us more of the peacock or the magpie?
Depends, I suppose, on the male. Maybe it has something to do with genes and their expression. Culture certainly has something to do with it. Lord knows there's a lot of male preen and bling and swagger; just look at any issue of People, Us or InTouch. And it's women who buy these mags, which are mostly pics of women tarting up for the peacocks. Or is it for each other?
Someone else will have to explain what's going on. I'm just a little old magpie (he said endearingly).