SFFP - You may be forgiven, it took me a while to get my head around (fnarr,fnarr for the Jurassic Park denizens who are coming out of the woodwork) a transgender friend.
Orientation and gender are 2 distinct attributes. A straight chap who has gender reassignment therapy will quite often become a lesbian, i.e. she still fancies women. Likewise (and much more rarely), if he was gay before the Op, she will be straight post-op. It's quite simple really.