I did not vote, because I think the question is framed in too limited terms for me. Nevertheless, I think the original question is a good post to stimulate thinking, and I have enjoyed the differing opinions and experiences.
My own thoughts are contradictory, like this debate - I agree and disagree with everybody. (Old Jewish story: a man comes to a Rabbi and complains about his neighbor. The Rabbi says "you're right." The neighbors counters with his side of events. The Rabbi says, "you're right." The Rabbi's wife, hearing this says, "But, they cannot both be right!" To which the Rabbi says, "You're right.") But everyone's experience has validity and is worthwhile.
I think part of the problem is a language issue; what we experience cannot be so easily put into the box of labeling. Or, to put it another way - to name our experience GID or GII is to (over)simplify it in order to talk about it. But it is important to remember that the label is not the phenomenon, it is an abstraction. (I like GII better).
Add to that that the "box" of "mental disorder" means different things to each person, and I doubt we will ever have total agreement.
To some people "mental disorder" does not fit their experience. To others it does.
One problem is that society seems to increasingly insist that you can only ask for help if there is something "wrong" with YOU. There has to be a flaw in you, but to get the best help, it cannot be your "fault."
An off-topic example of this is that in Los Angeles, California, to get help from Child Protective Services, a parent has to be labeled "bad". It did not used to be that way. In the 70's, an overwhelmed single parent could ask for help, without being blamed or labeled, and get help.
I wish we could get appropriate help without feeling blamed or made"wrong."
We cannot yet dispense with labels, most medical insurance demands there be an accepted labeled something wrong before you can get help.
But we have to get beyond believing the labels are more than convenient boxes, which are not real in themselves.
For me, it helps that there is a label, a box, that at least validates my confusing experience. I like GII better than GID. It helped me feel some less self-doubt to read the DSM and see parts of my experience in print. But I do not take the DSM too seriously - it is put together by committee vote, and it has major flaws.
I worry that people will use whatever label exists to justify negative actions against me and others whose experience is similar.
Is it a mental disorder? What is a mental disorder? An abstract concept. Not a concrete reality.
Does it help us understand our lives better? Does it help us grow and move forward? Explain ourselves to possible allies?
Being transgender is a lot of different things to different people.
I believe it is real, not imagined for most people. It causes most of us to feel out of sync with our bodies and with our surroundings, and that causes problems, which cause emotional distress - for most of us. But not uniformly.
I do not know if calling our experience a mental disorder helps or hurts. Probably some of both, depending.
But comparing experiences helps me, so thank you all.
Kendall