Women earn less money once they decide to raise a family due to lifestyle and schedule changes that don't affect their husbands.
Because of domestic inequality, women are often the primary caretakers of their children. This results in them having less time to dedicate to their work, ultimately resulting in them making less money than their husbands, who aren't expected to stay at home with the child as often.
The gender pay gap becomes most noticeable in a woman’s twenties or thirties, when they typically begin to have children.  The woman in a household is often the primary caretaker of the baby, so mothers are more likely to rearrange their schedules to accommodate raising their child while the father’s career and daily routine remains more or less the same. Women stop accepting extra hours at work and might even begin working part-time or from home to accommodate their new lifestyle changes. This results in women making less money on average. In addition, due to misogyny in the workplace, young women without children might not be considered for promotions to managerial positions, due to fears that they’ll eventually leave the company when they have children. Women also face an extreme amount of pressure from society (and sometimes from their own parents or families) to find a husband and have children. Men are sometimes faced with a similar pressure, but their decision to start a family is has comparably lower stakes than a woman's decision to have a family, since he will not be going through the physical tolls of pregnancy that may prevent women from being able to work at full capacity. Similarly, he is not expected to stay at home with the child as often as the mother might be, and thus does not need to worry about losing his job or readjusting his hours in a way that will result in him losing money.
Women understand that deciding to have a child results in major lifestyle changes, and motherhood is not something they opt into without careful consideration. Once again, people who claim women are often susceptible to succumbing to outside pressure to start a family are underestimating women's capability to make their own choices. In addition, the stigma against women without children who instead focus on their career used to be far greater. Our society is more encouraging of powerful, independent women climbing the corporate latter and becoming successful by any means necessary, even if that means not marrying and not having any children.
[P1] Women who decide to have children take more time off work or leave their jobs entirely to be the primary care taker of their child. [P2] Women are considered for promotions less often than men because employers know that women take more time off than men do when they have children. [P3] Women are more likely to encounter these issues because they're expected and heavily encouraged to have children.
Rejecting the premises
Rejecting [P2] This is not always true. If it was, we would see absolutely no women in manager positions. Cases might occur in isolated incidents, but this would fall under legitimate gender discrimination and therefore most likely doesn't happen very often.