Laura, I think your original post here hit the nail on the head. Many people think we choose to be transgendered. I for one don't remember getting to choose, but that's beside the point.
Not that many years ago being homosexual was seen as a choice. Many still think that, but more and more people realize it is just who they are. It is not a "lifestyle" but an in-born trait. Same with being transgendered.
Where the choice comes in is what you do about it. If you can suppress it or deflect it, then you can choose not to express it. However, many of us get to the point where we have to express it.
In my own experience, once I began to taste what it was like to be free I couldn't go back. After a month as Kate, my sister asked me to resurrect the old me for a party of hers. I told her that I just couldn't do it. It was then she realized that this is very serious for me and not just something I'm doing for fun.
I've never been suicidal, fortunately, but I told my therapist that if I was forced back to having to live as a man I would just have to kill myself. I just cannot go back. (I have this theory that we expand once we get out of the cage, and that's why it's so hard to cram us back into it.)
I don't know how you can convince your wife that this is not a choice, Laura, other than to hold your ground while expressing concern for her in every way you can. It is often the innocent bystanders who are hurt the most.
*hugs*
Kate