Transsexual
My favorite local T-girl occasionally does duos. I had inquired about it because the first time I saw her (she was only visiting at the time; she's now living in my area), she was advertising them with another T-girl she was traveling with. When I asked her about it recently, she said she occasionally does duos with a "very gorgeous TV" (as she put it) here in town. I politely refused, as my interest is strictly TS.
I guess it's a psychological thing: to me, a TS is very much a woman but with "that little something extra," while a TV or a CD is still just a guy in makeup and a wig (no matter how feminine the appearance or mannerisms). To her credit, she was very understanding, saying that I'm far from the only one to feel that way; she didn't bring it up again, and we proceeded to have yet another of our hot sessions together. 
(She also promised to let me know if she would be doing duos with a TS in the near future, which I plan to take her up on because it's something I've been meaning to try.)
So, again, it's a psychological thing. Which means that if I were to book a session with someone advertising as a TS, but turns out to be a TV or CD, I'd probably be even more pissed off about that than the fact that I was deceived in the first place. Don't know if that says anything bad about me for thinking that way, and I'm about as far from being a homophobe as you can get, but the bottom line is that I'm just not attracted to men, and a TV/CD is too close to that line for me to be comfortable.