I can accept that transgendered people are legitimate because the brain is extremely complex and human sexual dismorphism is a lot less specialized than most people think, but Furries who actually think they're animals on the inside are crazy.
Also if you're prepared to accept blogs on Tumblr as evidence that people can identify as other species then you'd also have to accept that people can identify as dragons, robots and fictional characters.
I'm actually cool with this.
As for the transgender thing and brains... yes, brains are quite complex. Yes, some people might have a brain fit for the opposite sex.
Now imagine we figure all this out completely, and we start looking at people's brains. There will be some trans individuals with the brain that matches their sex. Are you going to call them illegitimate? Are you going to tell them they aren't "really" what they identify as?
To me, someone's "identity" of any type can only be determined by themselves. There is no criteria or requirements except simply deciding it is so. They say who they are; you do not. If they find that a concept embodies "who they are," then they can associate themselves with that concept, be it a gender, animal, fictional character, etc. It depends far more on what they want to be
seen as and
associated with than how they are constructed, and there's nothing wrong with that.
If we start arguing whether someone is "really" this or that (on a metaphysical level), then we're arguing in circles through endless no true scotsman fallacies. No ifs ands or buts.