Being a tranny is a choice.
I agree, people don't choose to be gay or straight, the primal instincts of what makes you sexually aroused are pretty deeply rooted physiologically in hormones and neurotransmitters.
Choosing to live your life on a societal fringe, paying tens of thousands of dollars for medically unnecessary surgery is a choice. I'm not condemning anyone for that, either. But drastically altering your body and interactions with the rest of society to fill some emotional void or create an identity is still a choice. It's no different than the guy with head to toe tattoos and piercings, or the bodybuilder who spends thousands of hours in the gym and most of his money on steroids. Whatever the motivation, or emotional void to fill, it's still a choice.
All three may have some admirers or supporters, but all three are subject to ridicule behind their back, also. One, however, is painted as some courageous victim by a lot of the media.
I can call the body mod guy and bodybuilder guy annoying, attention seeking douchebags, and no one will flinch (probably even laugh and agree). If I even suggest the same about the tranny, chances are you're going to be labeled a bigot and xenophobe (or even nazi, etc).