I don't understand how people can think it's normal and not a symptom of a mental disorder.
@cjsummers :
The world we live in is not black and white. Who gets to decide what is normal?
The animal kingdom is full of animals who vary from traditional gender or sexual roles, its not just in our species. Its just that our species analyzes, it, talks about it, and judges other people based upon our own personal expectations.
There have been many scientific studies that recognize gender and sexuality on a sliding scale between male and female.
And in order to qualify for sex change operation, there is a long (as in at least 1 year, maybe more) psychological evaluation process one must go through and one must live as the opposite sex for at least a year or two before you can surgically alter. Many trans don't surgically alter their sex organs, they just alter their outward appearance with hormones, plastic surgery, and dress accordingly.