And I suppose the argument to be made against all the parallels to anorexia and body dysmorphia is that they just don't see it as an illness and find it deeply offensive when people refer to it as that, they just see it as them becoming who they really are and that society is trying to pathologies them.
My argument would be that anyone who is willing to go to the length of having such invasive surgery must be very distressed and they need help.
I'm also not sure if it's an illness as I don't think that the issue is necessarily with the person (though it sometimes is) but with the strict and oppressive patriarchal structures that make people believe that they can only behave certain ways and they'll be treat differently if they are the opposite sex.
However that structure ain't going anywhere soon so I do think the answer is intense therapy and in some cases surgery.
However I think that's true for a lot of mental illness (not all!!), people are expected to function in a deeply unfair and unhealthy society and when they become inevitably distressed it's called an illness, when it's a reaction to the environment.