I'm not really saying that we're the weaker sex, (not in a helpless way) but I'm just saying that women really are at a physical disadvantage to the man when it comes to childbirth.
I don't know if you've ever been pregnant, but it can be draining, and depending on what kind of job you are doing, maybe you can't do the same job while pregnant. A woman really can't do all that much while carrying around a bowling ball! The man goes on (passing up the woman) because this doesn't physically affect him.
Most men are physically (biologically) stronger than women. Maybe it's the testosterone?