It's not though. Women with children are less likely to be hired, whereas if a man has children, he is more likely to be hired, which effects the pay gap. The pay discrepancy is pretty significant when a woman works in a male dominated field and she is very often paid less, whereas a man is often paid more if he works in a female dominated field, which effects the pay gap. Women are generally given an incredibly low amount of maternity leave which leaves them with the choice between going back to work after a physically traumatic experience or losing their job, which effects the pay gap. Of course, that depends on whether she has a job to go back to, as many companies find ways to let someone on maternity leave go, which effects the pay gap.