[AnthonyMouse]

The U.S. Department of Labor looked at the actual reasons.[1] It's predominantly because men statistically choose to enter higher paying professions and specialties, work longer hours and take fewer days off. Accounting for such things leaves the pay gap in the neighborhood of 5%, and the DoL concludes that it could in principle explain more of it if they had better data available. Here's the last paragraph of the summary:

"Although additional research in this area is clearly needed, this study leads to the unambiguous conclusion that the differences in the compensation of men and women are the result of a multitude of factors and that the raw wage gap should not be used as the basis to justify corrective action. Indeed, there may be nothing to correct. The differences in raw wages may be almost entirely the result of the individual choices being made by both male and female workers."

The entire issue is just politics. For example, studies have shown that taller people make significantly more money than shorter people, by an amount that exceeds the unexplained pay gap between men and women based on the average height difference between men and women. Back of the envelope math then implies that women get paid more than men of the same height who do the same job.

[1] http://www.consad.com/content/reports/Gender%20Wage%20Gap%20...