I get mistaken for a man on a fairly regular basis. A lot of tall women suffer from this: Miranda Hart used the idea in an episode of her sitcom Miranda and Tamsin Greig mentioned it in a recent interview in the Radio Times. Are we some sort of reverse lady boys?
I've been mistaken for a man in India, North and South America, mainland Europe and Africa as well as here in the UK. Tall + blonde = American while tall + brunette = man, it seems. In France, a small child once pointed at me and said "Maman, un travelo!" (mummy - a tranny!). In Mombasa, a gang of kids trailed after me and my male travelling companion shouting 'Homosexuals!', thinking we were both men.
The problem is that people see what they expect to see, they don't look properly and fill in the gaps from scanty evidence because it's easier than closely inspecting everything and everyone they see. That would take too much time and effort for the brain. Unlike failing to discriminate between a rustle in the bushes and a tiger, there is little cost to getting gender wrong so no incentive to be more careful. Apart from getting a hard look from me. It used to upset me a lot as a teenager but now I'm used to it and can't (usually) be bothered to reply.
It's an example of confirmation bias.
In my case, they see something tall looming over them. If they're a shop assistant and sitting down, they look at where a head would be on an average size woman, don't see one and assume: man.
With confirmation bias, any further evidence that might disconfirm the initial hypothesis is ignored - hair, hips, make-up, breast size, voice. Information that confirms preconceptions or prejudices is favoured. Tall = Man. Tall = Man. Tall = Man.
There is a conscious or unconscious assumption still that women are petite, delicate things. A lady looks like Audrey Hepburn or Angelina Jolie not Miranda Hart - or me. I've had a charming gentleman lean out of his white van and inform me that I'm 'too tall for a girl'.
Sometimes they have the grace to apologise but very often after calling me Sir, they just carry on regardless even though they have recognised their mistake as I can tell from their expression.
Another response is a long conversation about how tall I am, whether my parents are tall, have I always been tall (yes, I was born this height), if I have trouble getting clothes/shoes/a boyfriend and can I please reach the jam down from the top shelf for them. It's like being public property in a way; people assume they can comment on my appearance (often coming up to me in public solely to do just that) in a way that they never would if I were black, for example (ooh, you're black, you're very black, are your parents black, do you like being black? etc etc etc).
Then there are the assumptions about my sexuality. And let's not even go into the fun I've had with short straight men over the years. No, I am not your personal Everest.
Ranting aside, the brain uses short cuts or heuristics in information processing. Heuristics are basic rules used to make decisions and judgements that work well in most cases (or we wouldn't have evolved the tendency to use them) but sometimes lead to errors. If height distribution is a bell curve with most women in the central bulge, then the height/gender heuristic works well. But for those of us at the lanky tail end of the curve, it fails.
In other words, it's not their fault for making a cognitive error, it's my fault for being freakishly tall (six foot and a tiny bit, if you've never met me).
So expecting people to open their eyes and look properly at me is not a realistic expectation: they have evolved to be fuckwits. Sorry, that's not very ladylike, is it?