From those who "don't get" my hair, I invariably get one of two responses. Either
1. They will mistake me for a man and apologize.
2. They will ask me "why" questions. "Why do you keep it so short? Why did you decide to cut it short? Why don't you care what people think about it?!"
For the first variety of people, it is always an interesting debate for me internally. Are they apologizing because they think I will be offended that they thought I was a man? Or are they apologizing because they are stuck in the gender binary and have realized something new today? In the former sense, I don't mind. I will admit, from the back, or when wearing sweatshirts, or sometimes just in general: I can look like a man. (Although clearly defining what makes a "man" vs. a "woman" is an entirely other debate for another entry perhaps.) I guess suffice to say just for now that I can look as society would expect a man to look (minus the facial hair - I swear, I'm working on it folks. :p) Unfortunately, this type of person will then spend at least the next 10 minutes trying to convince me that I am, in fact, a woman, and that it's very obvious to anybody who would just take a few moments to look. It becomes less about them making a mistake and more about me somehow feeling bad that I don't "fit in" with stereotyped gender roles.
It's the latter type of apology that is preferable. I enjoy people who have to face up to cognitive dissonance and admit that the way they view the world might not be the way the world actually is. This makes me happy, in a sense that I internally clap like a toddler who just learned to clap might.
Most often little kids will make the "boy/girl" mistake, and then we get into a whole other territory of cringeworthyness.
Young person: "That person looks like a boy!"
Older person (usually parent): "That's not a nice thing to say."
WHY NOT? It's an honest observation, and probably from that kid's world view, I do look like a boy. We get a bit into the whole "I apologize because you should feel bad for making us all look awkward now because your hair is short. Damnit, why can't you just fit in so I don't have to explain to my child that girls can have short hair?" I try in these situations to be as up-front with the kid as possible. "I do sort of look like a boy, don't I? Is it because my hair is short? I like my hair like this, that's why I do it."