I have an idea. Why not allow the man who's attracted to transvestites determine his own sexual orientation?
If not. Then the person should be considered homosexual -- alternatively bisexual -- as there's no definite point in the transition process when a man becomes a transvestite which you could unambiguously determine that it's not homosexual to have sex with it anymore. Is it after the first hormone dose, or the second? Is it after the transplants? And if so, wouldn't you be considered homosexual if you f*cked a very masculine man with just implants? Because genetically if you're born a man you'll remain a man for the rest of your life.
Then again sexual orientation centers around sexual attraction and not sex. A lot of straight men have sex with other men in jail.
I've heard a lot of credence given to the theory that the main physiological difference that defines who people are attracted to is how they react to and perceive pheromones. If that's true and the difference between straight/gay/bi is all in the nose, then it would be very difficult for a hetero or homo sexual person to be attracted beyond the superficial to someone who's had sex reassignment despite being anatomically male or female. maybe hormone therapy makes transsexuals produce opposite gender chemicals in their sweat?