Perhaps men and women are naturally inclined towards single-sex communities.
More simply, perhaps men are drawn towards content that appeals to men and women are drawn towards content that appeals to women.
Obviously there's lots of content that appeals to both sexes but you do reach tipping points. Two years ago /r/twoxchromosomes was an interesting and thriving community of "women on the Internet" but they lost visibility when the bitchy catfights of the Reddit LGBT and SRS stole the spotlight. "women on Reddit" are now far more on a war footing, with the end result that the broader website is substantially less friendly towards women.