The issue of transsexualism/transgenderism has to do with gender - and, not sexual orientation (as with homosexualism). People who are transsexuals not only relate to people of the other sex, but deep down they truly feel that they ARE members of the opposite sex. They feel that they are trapped in the wrong body. (Can you imagine how that must feel?!)
For argument's sake, lets compare a male-to-female transsexual (MTF) to a gay male (GM). Fine, as someone mentioned before, theoretically if the MTF is attracted to males (which would make sense, if the person considers themself to be truly female), if that person is pre-operative (still has male parts), then yes it would be a male being attracted to another male. BUT. Said person is not looking to have 'gay sex' with their partner. They identify as female. They don't consider themselves male, so they don't view it as 2 men having sex. The MTF is attracted to men who are attracted to females.
The GM, however, is very comfortable in his body. He likes being male and doesn't aspire to become female. He is attracted to men, and finds it a plus when those men find other men attractive.
Transsexuals are not to be confused with transvestites (drag queens/kings or cross-dressers). Transvestites enjoy dressing up in clothing that is generally associated with the opposite sex. They, however, don't aspire to get sex-change surgery. Transvestites can be both heterosexual or homosexual. (Oy vey, right?)
Like I mentioned before, transsexualism shouldn't be confused with sexual orientation. Like someone else mentioned, transsexuals can still be homosexual, just as they can also be heterosexual.
Then there are those who are intersexed, bisexuals, pansexuals, asexuals ... Oof!... We'll leave those for another day tho
Class dismissed!