Transsexualism in my opinion, can be a disease or just a wish to be the opposite gender. I don't really know enough about the subject to talk about this, but I've heard that some people have real difficulties adjusting to their gender. It's not they "want" to be a girl if they are a boy or a boy if they are a girl, it's that they can't behave like their gender.
Like I said, I don't know enough, but I once read about a guy who felt he was a female from the inside. It was said that he acted like a female, was addicted to girly stuff (like cooking, dolls, etc) and whatsoever. He did try to act like his real gender, but he couldn't. He stayed like that until he grew up, married, and got kids (I think, it was long ago). But after being unable to hold it in any longer, he got a sex change operation and changed into a woman. His "wife" began helping him (or her) adjusting to his new body.
What I am saying is that there might be really people who was born with these issues. You can't really know, but I think that as long as it's serious, or the person is having mental troubles because of it, a sex change operation can't be said no to, I guess, as long as it's serious enough.