In short, the theory is that women are extremely intolerant of low status men and try to avoid working in fields where they are prevalent, and particularly in fields where they are respected. It actually has a reasonable amount of predictive power, certainly more than the sexism theory.
I'll be blown away, however, if we actually follow the theory to it's logical conclusion...
[1] Roissy in DC is an extremely sexist, but insightful blogger. He didn't state this theory, but I've heard it described as the 'Roissy' theory since it follows his line of thinking.