This is a positively fascinating discussion. But remember folks, beauty is in the eye of the beholder...
Therefore, if you find guys / male genitalia attractive, and you are male, consider yourself homosexual, and vice versa. Doesn't really matter what others label you as.
All this "what people used to be" etc doesn't change whether you find them attractive enough, except if the idea of them in the past being a turn-off, thats the main issue.
And if someone is quite capable of accepting that a transexual (stop using the word tranny, it usually applies to transvestite), used to be male, but finds them attractive now, I'd say good luck to them, I wouldn't bother labelling them one way or another.
Consider this :
Hormones usually dictate whether we are attractive to someone or not - not DNA or anything else. So if you find a man's hormones attractive, and you are male, you are homosexual or bisexual if you find female hormones attractive too.
So, if a man undergoes treatment and produces different hormones, he is producing female hormones, so as far as attraction is concerned, you can consider yourself straight.
The whole penis / backdoor sex thing is just practical issues that have to be resolved when male-male attraction occurs.