Take two leftist ideal non-for-profit hospitals, identical in every way, with each hospital having equal numbers of men and women in each job position and paying each one the exact same hourly wage. The hospital pays 100% of every employee's health insurance. Every employee gets 12 sick days a year that can be rolled over but not taken as pay unless you are sick. The hospital shares any profit with the employees as a pay raise, but also shares any losses with corresponding equal pay cuts.
Now, take all the women from hospital A and switch them to hospital B, and replace all the vacant hospital A positions with the men from hospital B. The income of the two hospitals at the end a year will still be the same, but hospital A will be giving its employees a raise, and hospital B will be giving their all female staff pay cuts even though every single job position is being paid the same hourly wage.
Why? Health insurance premiums at hospital B go up due to OB/GYNE benefits changes the group risk and rates. Hospital B discovers it now has twice as many women out on maternity leave and getting sick pay, which it has to cover with either overtime, expensive temp agency labor, or permanently hiring more employees, and hospital A discovers it has no one out on maternity leave and their insurance premiums have gone down.