My hypothesis: Women can find sexual fulfillment more easily and hence spend less time fantasizing and hence spend less time in communities that talk about fantastical things that have little to no relevance to things they could realistically achieve in their lives (rationalists, atheists, libertarians, WoW players, etc).

I think if you talk with -- and more importantly, listen to -- some women, you'll find that the above is a projection, because, among other things:

1. Women fantasize plenty

2. Finding sexual fulfillment is MORE complex for a woman than a man

(Actually, just reading a few romance novels with a sufficiently open mind might suffice to illustrate these points.)