Susan Jacoby examines the gender imbalance among active non-believers. One reason for it:
The first and most obvious reason is that women, in the United States and every other country, are more religious and more devout in the practice of their religion than men. Public opinion polls show that this disparity affects every income, educational, and racial group—although it is much narrower among the highly educated than among the uneducated and the young than the old. African-American women, regardless of their level of education, are the most religious demographic in this country. This fact alone tells us that education is not the decisive factor, because although black women as a group are better educated than black men, black men are less religious. Space doesn’t permit a lengthy analysis of why women are more religious than men, so I’ll simply say that the greater religiosity of women means that both secular humanism and atheism are tougher sells to women.
She also ponders the historical causes:
Looking back further historically, it is just a fact that a great many founders of twentieth-century secular organizations, like the Center for Inquiry or the American Humanist Association, came from either a philosophy or science background—and these two areas of academia were particularly inhospitable to women before the 1980s. I should also point out that the few women who were engaged in science and philosophy had to work twice as hard as their male counterparts to maintain themselves professionally. They didn’t have the time to become involved in a marginalized secular movement. The energies of many of the smartest and most energetic women of my generation instead went into the feminist movement, which directly affected our everyday lives for the better. Personally, I’ve been an atheist since I was fifteen, but I simply saw this as something I was—not as something in which I wanted to invest my energies as a writer.