Take one factor, women earn less because of mid-career halts due to having children, like the father didn't also have a child. Women bear the brunt because we're expected to be the primary caregivers, and this hurts men too due to the "father babysitting his kids" problem of considering the father's involvement as secondary.
You can say this isn't a problem for her employer to solve but as long as we have no intention of upsetting the standard nuclear family gender roles men and women taking the default life path shouldn't consistently make one worse off than the other.