To those saying there is no "gender pay gap", yes there bloody well is. Men, on average, are paid more than women, on average, to the tune of around 75 cents on the dollar. This is not an apples-to-apples comparison of men vs women working the same jobs with the same experience and so on, but that doesn't mean that none of it is due to bias.
The apples-to-apples pay gap does appear to be real as well (to the tune of about 5%), but it's less certain. Different studies reach different figures, including no statistically significant gap.
So on to the OP's question.
The sterotype is that women with children always want to be the primary parent, and men with children don't really care. So there's both a lot more pressure on women to be the ones to, eg, take time off work to deal with the kids, and a lot more *permission* for women to do so (eg more maternity leave than paternity leave available). This means women with kids will work fewer hours, and thus make less money.
Similarly, the idea that certain skill sets are "women's work", and are easy and therefore not deserving of competitive pay, leads to women with typical female employment interests (eg nursing, teaching, secretarial work) to make less than men with typical male employment interests. Personally, I would much rather literally dig ditches all day than have to answer phones all day (yay, Asperger's)
And I might have more answers if I wasn't half braindead from lack of sleep.