Women make less because they are less productive. As a whole.
This. They generally don't take their jobs as seriously as men do, and are more focused on their social lives than anything else. Plus, they make lousy managers.
I think most of the perceived income disparity is simply because women still tend to be secretaries, nurses, teachers, and paralegals, rather than managers, doctors, principals, or lawyers.
Look at any school of math and science. Predominantly men. There have been concerted pushes to recruit women into these fields, but they just aren't interested. I work as a scientist, in a place with roughly the same number of men as women, but with only a couple of exceptions, the women do not have college degrees and are only able to work as secretaries or technicians under the supervision of the scientists.
The thing with saying women are paid less than men ignores seniority, skill, job function, productivity, etc. I suppose you could create a society where everybody is paid exactly the same, but as in Communist Russia, employee morale and incentive to work and overall productivity plummets.