I think this is one of those instances where you need to take control of a discussion. For some people, no matter what you say, they will simply try to put you down. Just walk away from these types. They don't want to know, tjhey think they know everything. They are just looking for an argument. To those that really want to know I would try to proced like this.
The few times I've talked about it, I have generally started by asking people if they can fully understand situations like, diabetes or pregnancy or being gay. I ask then, beyond their intellectual knowledge, to what extent can they really understand?
Generally, they respond by saying that they know people who have diabetes, been pregnant or are gay. To which I respond, 'And you know someone who is trangendered, me'.
Unless you are in the situation, you can only have an intellectual understanding. That is all I can give you.
I've heard the comparison with wanting to be anything. A rock star, a monarch, an animal. (This was before that South Park episode). I point out that people don't generally seek to be gay or have diabetes. It might be viewed by others as a disability, and to an extent, it is. But it's something you learn to live with. A diabetic just learns to live with the diet and other aspects.
On the sexual issues, I simply say that my persoanl life is no more a matter of discussion than their's is.
To the question, do you want to cut it off, I would ask, if you had an extra ear, would you want to have it removed? I point out that any decision to surgically alter myself is a matter for me and comes under personal.
From there, I just say that, I know who I am. I know how I feel. I don't seek to justify it to anyone, nor to impose myself into anyone else's life. I am and always will be the same person I am now. But my inner instincts of who I am is female.
Do you want to wear dresses?
I want to express my appearance to reflect who I am. Lots of girls never wear dresses. may don't own a dress. But I need to express who I am. How I do that is like everyone else, my own concern.