I'm happy to see all nos on the poll. I think that most importantly it's about freedom and self-determination. It's our sexy bodies, we can do what we want.

The problem is, that even in a perfect world, where freedom to be oneself was absolute, no abuse or discrimination was permitted, and people respected the laws and conventions, and never violated them -- it is still completely possible for everyone to not believe that you're the genuine article, and just simply have the right to act like it, look like it, and not have it brought to your attention.
I think that the scientific evidence angle is simply rhetorical, for the most part (though it actually does often move legislation, and have political effect as well), and an attempt to persuade others that our identities are not merely permissible, and protected from abuse, but legitimate, and true.
The prudent, practical matter, is freedom and rights regardless of what others may personally believe, and the most important thing, of course. We humans also need validation, acceptance, and to be taken seriously, though.