Righty ho, to clear one thing up, gender identity is a completly different thing to sexuality, although the two things are considered a scale and there for although not linked run in parallel
anyway, some trangender people don't identify a sexuality. perhaps the more simple sexuality for a trangendered person is bisexuality, because you can be either sex attracted to both sexes.
now a boy who identifys as a girl who will have sex exclusivly with girls would be classed as a lesbian, simply because the TG person in question, thinks, lives and identifies as female, and is called a she by her frends and family, despite the fact she has male sexual organs (or afte she has had sex reassignment surgery, just the XY chromasome in which you can't acually see). If the TG person in question has sex with other men, dispite the fact that the may or may not still have a penis which in biological terms would make them gay, they are not, they are considered to be straight.
I could explain for Female to Male transexuals, but the situation isstill the same only opposite.