There was a gender poll a while back that gathered some data on forum users
Also, later in that thread, anoek provided some data from Google analytics:
I feel quite unqualified to offer a compelling opinion, as I am not a sociologist, and have not really deeply studied this complex issue. However, I think the imbalance is largely due to cultural issues, and a sort of hangover from a time where many fields of interest, ranging from professions to hobbies/sports, were just so overwhelmingly male-dominated as a consequence of an unapologetically sexist and patriarchal society.
Although current society is still far from ideal on gender issues, things have certainly improved a lot over the decades. However, even in areas where attitudes have remarkably shifted more towards inclusion and even active outreach to encourage diversity, I think there is a cultural lag due to biases being deeply instilled by an older societal condition. In some cases, women might become discouraged to participate in a field due to the current underrepresentation of women, and hence making the process of equalizing participation much more gradual. Of course, a significant amount of sexism does still exist and surely plays a role as well.