It's no secret that men out-earn women. However, this income gender gap doesn't appear until women start having families - when women scale back work responsibilities to work at home. One writer in Australia has written an article justifying the income gap, arguing that women do not earn less for the same job - it is that men take on more hours and higher risk jobs, and so reap their just rewards. She writes that women make less because they study the social sciences, rather than the hard sciences.
Ben Stein echoes these feelings (thanks Rajesh) in his op-ed, writing that majoring in "African feminism in the 19th century...[or] Bulgarian poetry" does not lead to a lucrative career. Of course, these issues (like reproductive rights work) can have their own intrinsic rewards, and we are free to make our own choices, and live with those consequences.
Nonetheless, children still need raising, and women are typically the ones doing most of that work. An article in today's NYT suggests that while women are doing as much housework as they were 40 years ago, men are doing more housework. Women and men do equal hours of work, according to this study, if unpaid and paid hours are counted. Women who work outside the home work 19 more hours a week than women who only work in the home. Of course, all this housework is unpaid, and doesn't count towards retirement accounts.