Just because someone doesn't see things your way doesn't necessarily make them 'mentally ill'. To hold that belief is much the same as racism and homophobia.

Someone who feels the need to 'change gender' usually has to go through a lengthy process of interviews and counselling and living as a 'pre-op' before they are permitted by the medics to go through with what some people see as 'self mutilation'.

Changing gender is of no interest to me, nor are those who have undergone such a change. However, I do not think it gives me the right to sit in judgement on what they have done, or are about to do when those who have been trained in such matters deem that they have made their decision rationally and in an informed way.

On a personal level I find it difficult to regard a transgender person, be they pre-op or post-op, to be anything other than the gender with which they were born. Okay, they may think like the opposite sex (though I somehow doubt that!) and they may feel more comfortable adopting that persona - they may even cosmetically appear to be a different gender... but that's as far as it goes.

As 'Guest' said, it's their business, not ours.