I'm writing this post because it seems so many people misunderstand what transgenderism is and I get tired of repeating myself.
First, I hear a lot of people saying that these people are "mentally ill" and the to see a professional. Right on both counts, but not for the reason you think. Feeling you should be the other gender IS defined as the mental illness. The cure for that illness is a sex change. You get a sex change by first being assessed by a psychiatrist. So by saying that, you're basically recommending transgendered people do what they what they are quite willing to do, and most likely already have done.
Second, being transgendered is not the same as being homosexual. Transgenderism not about about sex, it's about sexual identity.There are plenty of people who go through a sex change only to be labeled as gay - In other words, they are attracted to the same gender they changed to.
Third, there are very good reasons for not going all the way with a sex change. For those going from female to male, the surgical options are very risky, seriously expensive (40 -80 Grand), and often result in an inability to experience sexual pleasure. Even a full hysterectomy is not a surgery to take lightly, and if the person has any surgical risk factors doctors may refuse to do one that is optional (As long as they continue taking testosterone, FTM's don't have periods and can't get pregnant anyway). It's not that FTM's who don't have surgery are happy with the bodies they were born with: For most it comes down to choosing the least evil.