If this was the case, why do the societal roles and presentations associated with each gender vary so much throughout time and place ? Some decades ago, little boys tended to be dressed in pink and little girls in blue. Throughout the ages, men wore make-up. And let's not even talk about dresses and pants. What make you think thatouryour set of conceptions of what is feminine and masculine is somehow the right, "natural" one ?
Some aspects of gender roles can be somewhat linked to objective factors (e.g men being more often than not the dominant sex throughout societies). But very few of them can be explained by it (e.g women warriors being the exception rather than the rule). And virtually none of them can be justified by it (e.g women needing protection when they are pregnant... duh).
Let's just say there's a difference between not imposing gender roles upon one's children, and using them as a social experiment.