Ever since I was a child, I've been filled with the certainty that I wasn't like other boys. When other boys wanted to play baseball, I felt that bi-pedalism was just not right for me. When told it was bath time, I felt only dread that my protective mucus coating would be stripped from me and I worried yet more about the salt content of the bathwater. When teased by classmates, I tried to retract into my shell only to be frustrated to discover that I didn't have one. But perhaps the most cruel insult was the discovery that did not have both a penis and a vagina and the gonads to match. Fortunately, with the assistance of a support group and a surgeon sympathetic to my plight, I am scheduled for experimental shell transplantation as soon as a suitable donor is found. If you think this is hard to read, think about what it must be like to know that you've been born into the wrong phylum.
Except you can't think about what it is like to be transphyletic, not only is it philosophically problematic, it is a patently absurd claim to even make. So too are the claims of people who believe themselves to be transgender; to say that they are mistaken is generous because to merely be mistaken assumes there could exist a world in which they were correct.