The body of a boy, but the mind it seems of a woman. If you put on a uniform and called yourself a policeman, you would be wrong because, of course, there is far more to being a policeman than the uniform. There are skills and qualifications and an official process to go to beforehand. Indeed, there is also more to being a woman than wearing clothes. You also have to think of yourself as a woman, and what is more you have to do so just about all the time, every waking moment. To maintain something for that long without believing it to be true, without knowing it to be true, is nigh on impossible and certainly very damaging. We've seen in the past that people who are forced to be what they are not, it never works out well.
The problem in this case is that you're unable to separate "social" gender, which is a set of behaviours and clothing and appearance and other, somewhat intangible things, from the biological side. Is there really a link between this? Why should there be? Indeed, given the well-established existence of the "intersex" at a genetic level (XXY, XYY, XXX and so on), it should be clear that there is more to biological gender than man and woman. For that matter, if, as pixie did earlier, you treat intersex as a third gender, or at least as distinct in some way, then what choice have intersex people? They can identify as either a man or a woman and yet, presumably, this would be wrong , because they are being something they are "not".
On the other hand, societies exist in which a third gender is well-established. From a traditional Western point of view this third gender is probably closest to men behaving something like women, but to label them as such would be insulting and inaccurate -- because in their society, they are neither and the roles associated with each gender allow for this third path.
And, again, the way in which men and women behave has changed over time even in this world. Until the early 20th Century, young boys would wear dresses until maybe they were as old as eight. Back then, clearly, the dresses they were wearing were not seen as gender-specific. Now it seems that has changed -- as we have seen on AB recently, the thought of a young boy in a tutu is controversial.
But the overall picture is that social gender is something that society has imprinted on top of biological gender. And then, in the odd cases, society has got it wrong with some individuals. You are born, the midwife shouts "it's a boy/ girl", and you are brought up usually according to that gender. In most cases this seems not to matter much. Biological girls are generally comfortable behaving that way, biological boys likewise.
But in some cases it doesn't work out that way, and for various reasons it seems that the imposed social gender matches up with the person's physical gender, but not their own image of themselves. When that happens -- and it does happen -- we should respect that, and accept it, and let them life their lives as they so wish.