I think the physical act of pregnancy has a lot to do with being what society defines as a women. It'd be interesting if men were able to carry children like women do. How would the world change?
I actually had written a big old post relating to this a few days back, but lost it. I'll do my best to recreate it:
Cultures are tied to the lives of the people who practice them, and as such behave kinda like living things... they're subject to laws similar to those of Natural Selection in that they are created, grow or die, splinter off from one another and evolve into new cultures, and even go extinct. The Gender Roles which define what it is to be a Man or Woman are a sort of cultural Defense, which Human populations invented over time to help them and their Cultures survive.
Allow me to explain: one of the big reasons that Women in many cultures have historically been limited to domestic tasks was a matter of survival. In historical environments, where Humans had higher mortality rates due to the lack of knowledge of how to treat injuries and such, keeping your population level stable could be difficult... and in this regard, Males are much less valuable than Females. In communities which didn't have any sexual division of labor, and equally exposed their Females and Males to injuries from tasks such as hunting, warfare, heavy labor, etc., the loss of adult Females was felt harder, since it meant the community could have fewer pregnancies, and it would thus take longer to replace those lost lives. Meanwhile their neighboring communities, which started limiting Females to less risky tasks, would lose more of their Males, but could recover those lost much more quickly with a larger population of Females, since one Male can breed with more than one Female. Thus, cultures with Gender Roles that lost fewer Females grew and expanded more quickly, ultimately winning out in the Cultural Arms Race. This, along with a sharp drop in mortality rates due to technological advancements, was so effective at allowing human populations to grow that we're now facing an overpopulation crisis as a species. Thanks a lot, sex-based division of labor... real helpful there!
On that note, the Gender Roles we still uphold are pretty much unnecessary at this point; we don't have trouble with mortality that requires us to shelter our women in a domestic bubble, while men act as soldiers and laborers and such. In fact, I could see a lot of great arguments for formally adopting more fluid notions of Gender... for instance, accepting Homosexual couples as Families, and not making it so hard for them to adopt children, which could really help deal with the millions of children without parents who are raised within institutions.
On another note, I thought it was pretty interesting to learn that Marriage customs may have a biologically inspiration. Many of the apes we are most closely related to practice something called Female Exogamy where, to help avoid incest, females leave the social group of their birth and join the social group their mate belongs to. Every species that lives in social groups practices either Male or Female Exogamy. Our Human ancestors probably inherited this practice from whatever common ancestor we shared with these apes, which eventually evolved into the cultural marriage practices we see in much of the world today; women are expected to marry into a man's family, and symbolically join it by adopting their surname. Kinda neat, huh?