Just a thought but perhaps it's because female SF authors were more concenred with human issues and characters and men are more concerned with technological issues and social backdrops. The former being more timeless concerns, the latter being far more likely to date. A gross over-simplification perhaps but there might be a slivver of truth to it...
I had a similar thought Egg. I figure that generally men are more interested in technology hence a bias towards SF and women are more interested in spiritual stuff giving a bias towards fantasy. Gross generalisation of course, I'm just talking about general trends. The next question though is, is that due to nature or nuture? My tendency has always been that it is largely nurture, society tends to give meccano sets to boys and dolls to girls.
That said there have been some interesting experiments I saw in a documentary on the box a while back. In one they dressed young (around 2 or 3 years old I think) boys as girls and young girls as boys and then had an adult (who didn't know them) try to entertain them (one at a time). Most adults tried to give toy cars and diggers etc to the apparent boys and dolls to the apparent girls. Clearly society's conditioning there. But the interesting thing is that the kids weren't interested; the girls didn't want the cars they wanted the dolls and the boys wanted the cars. Not massively conclusive (maybe society had already conditioned the kids) but interesting.
The more interesting experiment was a with a group of monkeys presented with a collection of kids' toys. The female monkeys (even the very young pre-pubescent ones) all chose dolls and the male monkeys all chose mechanical toys, things that had moving parts and stuff. Again not conclusive but interesting and it really was that absolute the females only took dolls and no male took a single doll. I no longer remember the exact conclusions the scientists came to but I think it was along the lines of male and female brains being wired somewaht differently.
So now I'm not sure