Hmmm...for me, sex is something you DO, gender is something you ARE. Thus, heterosexuals enjoy having sex with those of the opposite gender, homosexuals with those of the same gender, and bisexuals with both. For me, the word "transsexual" is an incorrect term and should always be replaced with "transgender." I suppose if I were to define the former, it might be something along the lines of "someone who is confused about with which gender they prefer to have sex."

I'd comment more, but my migraine-addled brain really isn't up to it at the moment.