Neurologists have tentatively concluded there is a real and tangible difference between the brains of men and women, which can explain much of the symptoms we call "gender dysphoria". This is not a factor in people wishing they have only one arm.
I mean, pschyatrists should determine whether these people are best treated or whether it's best to allow them to go through with such a procedure, but it is a different kettle of fish...
But isn't the difference between folks with one arm and folks with two arms the most fundamental division in our society?