Your brain doesn't determine sex? You can him and haw with "identity" all day long, but your biology is fixed in this scenario. It's really quite simple. Whether one accepts the truth of their nature may be a question or problem in and of itself (rooted somewhere in psychology I can't presume to tackle), but alas, the chromosomes don't lie.
Now, there are people who are born both, or biologically mutated in such a way that they have a genuine crisis on their hands, considering their both their gender and sex. So I understand the identity crisis involved there and harbor no ill will on a decision to relieve such a situation.
As for the healthy folks who simply don't want to be what they were born to be, what can I say? I wanted to be a pilot, but I'm not. So I did something else. Perhaps I'm just a simpleton on these matters, for those that struggle in it, my apologies. I just don't see it as anything but pride, rebellion, and/or mental illness needing addressing.