Why do people get off on bondage or being urinated on?
I don't think you can choose to be turned on by kink, either. However. generally kink is private, while gender is public, so it's a bad analogy. Trans* people are not typically sexually aroused by their trans* state, to my understand.
I don't think that argument, that no person would choose to be gay, is very strong and I actually think it's kind of insulting, like the lifestyle is so undesirable;e and looked down upon that people must not have a choice.
The lifestyle is considered undesirable, and trans* people often lose family and friends over it.
Now, I figure most people never made a conscious choice, but that's not saying no one has ever made that choice, or that if they did they made a stupid choice.
I agree that the range of human society and experiences are so vast that there are probably a very few who actually choose to be trans*, but they are the exception.