geckosfeet is right -- there are cheap labour pools, and one of them, historically, is women.
Why they exist is the question. Immigrants are an example: today, their disability may be status, and they are vulnerable to exploitation if they don't have it. Historically, they often had poor skills in the majority language, etc.
Women's disabilities often relate to childrearing responsibilities. Women cannot make themselves available for employment that comes with various demands: overtime, travelling, infexible schedules, etc.
Perceived/ascribed characteristics also have a negative effect on other members of the group: immigrants with status and excellent English, women with no childcare responsibilities. Stereotyping and discrimination abound.
This is actually a fascinating subject that I don't think anyone has come up with a really good explanation for. In a "free market", being a woman or being an immigrant would not affect a person's attractiveness to an employer, or ability to find work at wages equal to, say, a native-born man's. If economics has no alliance with anything but money, why do workers' personal characteristics come into their wage equations, and why do these cheap labour pools exist?
There is a real difference in the case of women, though. The fact that any woman may leave a job at a moment's notice to devote herself to childrearing, after the employer has invested in her, or make demands on the employer associated with childrearing that a man doesn't make, certainly has long influenced women's attractiveness to employers.
Yes, women (and other low-status / stereotyped workers) are willing to accept lower wages, to at least some extent because they are discriminated against in higher-wage positions and occupations. So yes, they accept lower wages because they are offered lower wages.
Why, is the question: why do, say, native-born white men in the US not have to take part in this race to the bottom to the same extent in order to get work?
I'm really just mumbling here, but I'm hoping that what seems to me like maybe an unfortunate misunderstanding can be overcome -- or even, if there is real disagreement, it can be investigated in perhaps a more evidence-based way.