I've been mistaken for a woman several times over the internet, though in other places where I use a stylized, long-distance shot of Tachibana Ukyo (womanliest manly man I've ever seen) as my image of choice, rather than the Demi-fiend. It's all in the name ending in -a, I think.
I would think that most people prefer to have an avatar that--gasp of alarm!--represents themselves in some way. Break yourself down into a few nouns and adjectives; mine might be (genius, awesome, man, shameless egoist). I chose the Demi-fiend as my avatar because he more or less embodies those concepts (or at least most of them) in that he, in his game, is basically the only person in the world with the power to matter, to make things happen according to his will, and nobody is qualified to even begin to say he is wrong. Kind of an appealing image, to me. Also, you gotta love the tats.
So females for whom 'woman' is an important descriptor will probably choose a feminine (though not always female) image to represent themselves. And for those who 'woman' isn't that important, but who do consider feminine traits central to themselves, a feminine (though not always female) image is still likely to be a more appropriate (and apt) choice.
Like it or not, everybody judges on appearance (blind people notwithstanding--they judge on sound, of course. Unless they're also deaf. You get the idea). And when you can't see the person themselves, you judge based on the appearance they bring to the table to represent themselves. (And, when they don't have any appearance, such as on boards that don't allow avatar images, you look to their name for a first impression.) So you may as well represent something you consider to be related to yourself, yes?
Plus, anyone can tell that I'm a girl by my username (I even got "cats" in it too).
