Many people feel that someone has an obligation for someone to give money or subsidies to a woman just because she has chosen to reproduce. Usually this is justified by calling her a victim or claiming that such subsidization is good for society.
In reality, private enterprise employers factor the risk of having to lose (and then be forced to reemploy) a woman when they negotiate the pay rate. Women are more likely to take maternity leave, hence are a higher risk, therefore are less valuable employees, and therefore are worth less money.
This is hardly fair on women who don't want (or can't have) children, but the employer doesn't have this information. The employer can only act on the information available. Hence women get paid less.