I've read studies in the past (don't have link) that women tend to take more sick days and miss work more frequently than men.
And of course there is maternity leave which normally isn't a male thing.
All of which are covered by other aspects of employment contracts (see "sick days" and "family leave"). They should have no bearing on hourly/yearly salaries.
Also, not all women take sick days or have children. I have taken 3 sick days in the past 4 years, have no children, don't like children, won't have children. I should not be paid less by default because other people with a similar set of chromosomes do.
That said, I do not have a problem with seniority rules that take into account amount of time actually worked. If two people have been at a company for 10 years doing the same job, but one has worked 20 hours a week and the other has worked 40, then the latter employee has produced more during his or her employment time and has therefore earned a bigger raise and more seniority than the former. Sorry to those who have a problem with that, but it's pretty cut-and-dried.
Besides which, if a company has two people wanting a job, one is willing to work for 25 thousand a year and another wants 35 thousand, why can't the company hire the 25 thousand one?
They can, as long as they're not offering the person with two X chromosomes $25,000 instead of $35,000 because she has two X chromosomes. Gender is not a "qualification" to be legally taken into consideration any more than race, religion, height, hair color or shoe size.