GUEST,Max Reiner if by mentally ill you mean that they are suffering mental anguish by living their social lives with the apparatus of a gender they were born into but knowing in their minds that they really of the opposite gender then you are right.

If, on the other hand, you believe that they are mentally ill for feeling that they were born of the wrong gender, then you are sadly misinformed and I suggest that you educate yourself on the issue.

You can start by reading some scholarly literature on the subject: transgenderism and the concept of gender