Women's average pay is less than men's. One reason for this is that the average pay in jobs dominated by women is less than that in jobs dominated by men. Another is that, within a given job, men usually earn more than women, which is when women can actually claim wage discrimination - but that's another story.
The answer (mostly): Women came first. It's devaluation, not queuing. (This is the wagon to which Matt Huffman and I, apparently wisely, hitched our previous work.) This is especially true in the later decades.
Note this doesn't mean devaluation is why women get paid less overall. This can also happen for other reasons, including wage discrimination within jobs. But the conclusion is important, because devaluation is much harder to sue for under current law, which requires individual acts of discrimination either in hiring (the labor queue) or wage setting (within-job pay) -- both of which are hard to document.
Because it's not about individual discrimination, policy-wise, the devaluation thesis brings us back to "comparable worth" -- the legal regulation of job pay based on the qualifications and requirements of the job -- which I rather weakly advocated when the Democratic majority held hearings on gender inequity way back in 2007. (Contrary to Obama's assertions, this would be the real equal-pay-for-equal-work). Comparable worth requires government intervention in wage setting, making it the bogeyman of anti-discrimination policy.

Cross posted from the Family Inequality blog.