In one sense, atheism is about as gender-neutral as it gets. Lacking god belief is something that can and does apply to women and men alike. Organized atheism and atheist activism, on the other hand, sometimes seem to be more attractive to men than it does to women.
Daylight Atheism recently conducted a brief reader poll and learned that roughly 60% of those who voted in it were men. I found his discussion about what this may mean an interesting one, mostly because one of the four possible explanations he considered was something I have been thinking about for some time.
Hypothesis #4. Something about the subject matter or content of this site, in general, appeals to men more than to women, or makes women feel as if they're less welcome than men.