Sexual dimorphism was a crucial biological development of many billion years ago (I'm guessing) with rather profound implications for how not just people, but plants and animals, inherit and pass on traits. Race is not. Having different races serves no biological function -- just cultural and economic ones. (Actually i don't believe in races as biologically distinct.) We know races are socially constructed because we have the history of that construction (see theodore allen, for example, and Noel ignatieff). I don't think the same can be said about sex. Without sexual dimorphism --not trimorphism or sixteen-morphism-- we would not exist.What we make of it is flexible,sure -- I have always agreed with this point! HOW flexible we don't know. If women can give birth routinely at 55 -- or lose interest in giving birth -- then obviously it will be less important than it is today that female fertility declines faster and more abruptly than male fertility.