Polymorphisms for complex, polygenic traits?

Hi there,

Looking at a list of examples of polymorphisms, nearly all of the examples seemed to be for very simple phenotypic traits (color variations and so on).

Can anyone explain to me (in layman's terms! I'm no biologist) how sexual dimorphism ends up being a big suite of physical and even behavioral differences, all within the same species?

Is it because of the fact that different sexes have a whole chromosome different?

Is it because most of the differences that we see between the sexes are not due so much to genetic differences as to differences in gene expression in response to differential sex hormone exposure?

Is it both of these factors? Other factors as well?

Thanks!