I don’t have a lot to say about the mechanics behind it, since I’m not privy to them, but my former project GWAP is testing out a gender guesser. Based on your preferences for 10 pairs of images, it seems to achieve decent accuracy guessing your gender. At least of the 10 or so times that I took it, it got it wrong twice.
GWAP's new gender guessing game
I took that test 12 times randomly clicking and it came up female each time. I’m a dude