There are few arguments that baffle my mind as much as this one. I see it thrown around rather persistently as an excuse for sexism in the world. It is an moronic argument though and I'm going to tell you why.
1. Of the top 10 highest payed college majors in the US, every single one of them are dominated by men. In most of them the majors are only filled by about 10% women.
The 10 bottom majors are dominated by women.
2. Women are less likely to be consistent workers than men due to pregnancy and child care. Salary is something that is built up consistently over time, and when you don't have consistent work, it makes it very difficult to have a consistent rise in salary. Men are twice as likely to work 40 hour weeks as women.
When you actually look at the statistics for single women who do not have children , women earn 96% of what men do. Holy shit