[karlkeefer]

Perhaps it is due to "family costs" being carried disproportionately by women.

Leaving early to pick up kids, staying home when kids are sick, etc. Even though typical gender roles are becoming less typical, they may still result in differential output from the perspective of an employer.

Thomas Sowell (a while ago) showed a dramatic difference in salary between all women, and women who had never married, suggesting that a huge component of the pay gap was due to household and family expectations.

I imagine there is more recent research on this exact theory, but I'm not aware of it.