Biological fact, if you have sex with someone who is the same sex as you, you are homosexual. I don't care what bits he has paid a surgeon to remove or redesign, he is a man. If you lose an arm/leg as a result of misfortune you were still born with two. A cat can have kittens down the dunnie but it don't make them toilet paper.
I'm not into Trannies in the slightest, but your rather small-minded point has three main holes - pun, (not initially, but I'll happily put it down to intuition) intended.
Firstly, chances are that they're legally a woman, and considering "homosexuality" is as much a legal definition as anything else, I do think this is very relevant. Also, transsexuals are biologically as much of a woman as any who've had their womb and ovaries removed for medical or other reasons.
Secondly, I'm sure it's purely the female attributes of transsexuals that our more experimental comrades are attracted to and would probably run from any set of cock 'n' balls thrust in their face. Therefore, I can't see what homosexuality is present. Just because they're surgically created tits and pussy, doesn't make this any less valid. Perhaps you're not a fake tit man, but it would be ridiculous to say that someone who finds a great pair of enhanced breasts very attractive doesn't actually like tits because those ones aren't 100% real and unaltered. Find a woman's face, for example, that isn't.
Thirdly, as Firebird says, just because you've experimented with a member of the same sex once or even more doesn't make you homosexual, or even bi-sexual. Only they themselves know how they really feel, but unless a bloke is at least reasonably regularly sexually attracted to men for being men and their male sexual parts, (not people born as men who now have completely female physical parts, and a feminine appearance) more, as much, or at least in a comparable degree as they are to women, then they can only be classed as straight.
