First, if women were paid only 75% of what men of the same level of experience and performance were paid, then any employer could hire four women instead of three men for the same money and gain a decisive advantage in production costs over competing firms. Employers are in business to make money. If they were discriminating based on sex, they'd be seriously hurting themselves.

In 1991, women without children earned 95% of what men earned, while women with children earned 75%. The difference isn’t between men and women, it’s between women with children and those without. Why? Because some careers, particularly in technology and the sciences, rapidly advance and require continuing education and exposure. When a woman has to take time off to bear and care for a child, she’s naturally going to be surpassed by her coworkers who are still immersed in the field, gaining more experience and knowledge, and thus getting promotions and raises. Furthermore, in a constantly and rapidly advancing field such as technology, competition incentivizes an employer to hire those who can stay on for long periods of time without break so as to gain an advantage in the respective market. And there’s nothing wrong with that.