Frank is actually correct according to the BLS article that was linked.

"Women and men tend to work in different managerial and professional occupations. In 2005, among professional and related occupations, for example, women were much less likely than men to be employed in some of the highest paying fields, such as engineering and computer and mathematical occupations. Instead, women were more likely to work in lower paying professional occupations, such as education, training, and library occupations."

It does appear that there is an element of choice that contributes to lower pay.  As was mentioned, total compensation may actually be equal because of benefits provided by the employers may account for a greater portion of the total compensation.  This is true for myself.  When I worked in the private sector about 95% of my compensation was salary.  Now that I work for government over 25% of my compensation is from benefits, but I do work fewer hours and spend more time with my family.

"Women are more likely than men to work part time — that is, fewer than 35 hours per week. Women who worked part time made up one-fourth of all female wage and salary workers in 2005. In contrast, just 11 percent of men in wage and salary jobs worked part time. These proportions have not changed much over time."

It also appears that women as a group do work fewer hours.  Typically those who work part-time receive less compensation than those who work full-time regardless of any other factor.