My company paid my male co-workers a start salary of $8000 more than me. They had less experience and less education. The answer was they interviewed better.
Now I have my MBA they told me to take on additional work and in 6 months they will review my requested for a raise after receiveing the MBA they helped pay for with the tuition reimbursement program. Yet each day they pay is higher than mine for the same duties.

I am a man and I am sure if I would be in your situation, I would have threatened to quit already. Did you do that as well? Maybe that´s the difference between men and women. Just guessing.
edit: ps. if this hypothesis (men being more aggressive negotiators) would the true, the downside would be that men also have higher chances than women of ending up with nothing (if the threat gets dismissed), which studies (don´t have reference) seem to confirm that men´s success at work-distribution curve is more dispersed than women´s.

I think at times women demand a so called "work/life" balance - time with the family, children, - and also expect to have time off when they're pregnant etc.

I'm pretty sure those factors affect somebody's work performance... and the fact that those potential in every woman, discourages the employees more.

It's like buying insurance, the more risks you have, the higher your premium, only in this case, women get paid less.

I totally agree with fishball.

I think men are better at getting the job done but women are much more intelligent then men.