I have to wonder if that is because sexually assertive women (more specifically, female 'tops') are not considered abnormal, but submissive men, are; so much so, that a woman who might occasionally fantasize herself as a male during sex would have no reason to go to a therapist. We also have to remember that the DSM is not by any means perfect; for example, they have expanded the diagnostic criteria for ADD so far that right now, the majority of the general population can be fit into that diagnosis.