In previous posts I have written in support of affirmative action under the assumption that "men and women are equally capable of succeeding as professional astronomers. There is no inherent (intrinsic) difference in mental capacity, creativity, ability to learn, or any other factor that plays into the success of an astronomer." However, after digging around a bit, it turns out there is a difference in mathematical ability between men and women (Hyde, Fennema & Lamon 1990), and it's often cited as a reason why there aren't many women in "hard core" science fields. But it doesn't really work that way; the achievement "gap" is not at all what some would imply (h/t Slate).
Here's an insightful and witty slide presentation put together by computer scientist, Dr. Terri Oda, that should help explain things (see also this comprehensive review article by Elizabeth Fennema). Seriously. Go ahead and click through it. It takes less than 5 minutes. Then share it with anyone who attempts to explain the gender gap in the sciences on differences in the inherent math skills of men and women.
As Terri concludes, the only people who are bad at math are those who blame biology for the lack of women in astronomy.