If a gay and a lesbo started dating and had sex changes, then they wouldn't be a very gay couple at all. They would be just regular people with inverted sexes. 
Attraction for a transgendered individual while being transgendered themselves would require studying. Where the attraction comes from, regarding each of them. While a transgendered individual is considered to be, by all intents and purposes, the gender he/she has picked to be his/her identity, you gotta take into factor that a relationship within that gender would require for each significant other to be attracted to a certain kind of person in a certain degree. Does the physical attraction matter in this case? Are they in love with what's inside, without taking into account the outside? Some people would date transsexual straight people, but not natural gendered people. Everyone has a way of thinking, and maybe quite a few factors would matter. Maybe the inclination to be attracted to a certain individual is more about some attractiveness factors than others. It all comes down to Freudian stuff, honestly.