The current tax system is sexist.
According to
this paper by Alberto Alesina and Andrea Ichino women should be taxed at a lower rate than men because the women’s labor supply is more elastic than men’s labor supply. For those of you who aren’t econo-nerds that means when wages increase by x amount women will supply more extra labor than men will, and when wages decrease by x amount women will decrease the amount of labor that they supply by more than men will.
Politically it isn’t going to happen. Economically it makes sense.