Is there a gender imbalance?
No one knows for sure. There is no source of data that can tell us. The common reasoning seems to be that it's likely there are significantly more men on the SO network; the magnitude of this difference is similarly unknown.
Does it matter?
In a general sense, StackExchange is supposed to be a friendly, helpful environment. As some of the questions and comments around this question show, we still have some more work to do in that arena. If our culture here is pushing women away from participating, then yes it matters very much. It is a symptom of a problem in our community and we should take it seriously.
More particular to our site, it matters because viewpoints matter when discussing politics. I'm a political scientist - my training predisposes me to thinking there are objective, evidence-based answers for questions about politics. Similarly, this site exists on the premise that we can provide factual answers. I recall reading that while we discuss politics here, our answers themselves are not political. Some have said that in this environment gender is unimportant.
I disagree. The experience of political science has been that gender matters a lot, even when discussing factual things. For one thing, gender influences the kind of questions that we ask. In my field (political theory) it's easy to see how gender has influenced past philosophies. By far the largest growth area in political theory has been critical theory, including feminism, which addresses the role of gender in our theory. There has been substantial interest in the political science community for increasing our focus on the role of gender, both in our work and in our own community.
Gender also influences our answers. Typically not in overt ways, but covertly in the kinds of sources we think our reasonable, the kind of facts we deem pertinent to an answer, and the kinds of statements that seem acceptable. In all of these personal judgments, gender is important.
What can we do?
I don't know. We currently don't do any routine active promotion of this site. However, I would encourage anyone to actively promote Politics.SE in their own corner of the universe. If you feel strongly about this issue, perhaps promoting Politics.SE at your local college's department of women studies would be helpful.
The most important thing we can do is tend our own community. There are a lot of issues beyond our control that impact users' experience and participation; let's not create more problems by pushing people away.