The first three categories are defined in term of whom the folks in that category want to go to bed with. The final category is about who the folks there want to go to bed as even when they go to bed alone.
First of all, I have been in many support group meetings and only rarely, believe it or not, does the subject of bedroom sex come up. It is simply so far off topic that we rarely venture there.
And I have never seen it hold the attention of the group more than briefly.
Questions of identity, of acceptance or rejection, the practical problems of transition or of dealing with prejudice and discrimination, yes, those come up all the time. But rarely does erotic sex emerge as a topic.
It is impossible to predict from knowing that someone is trans the sort of person they will prefer as a sexual partner, if, indeed, they even want a sexual partner.
(Many of us aren’t much interested in sex. I’m not very interested myself, although I wouldn’t mind a good cuddle and lots of affection!)
I know trans women who strongly prefer women, cis or trans, as partners. I know trans women who strongly prefer men, cis or trans, as partners. I know trans women who could be happy with either. We are all over the map.
So to define us in terms of whom we would like to have in bed with us the way one defines lesbians, gay men, and bisexuals makes no sense at all.
To recap, being lesbian, gay, or bisexual is about what you do during those brief intervals on a given day when you are having sex. Being transgender is about who you are are the rest of the day.