Well there is one pregnant man (http://www.advocate.com/issue_story.asp?id=52664), but what you've said is interesting and I've wondered about it. Do you think the aspect of physical sex which leads to hierarchical genders is the ability to gestate? It's a problem with counterfactuals about how aspects of female physiology are perceived as flawed due to gender hierarchies. It's difficult to know whether a world in which the sex with gender priviledge also gestated young cold exist.