One of the best indicators of sexual orientation isn't what you do, it's what you fantasize about while you're doing it. The classic example is the straight man who has sex with other men while incarcerated. Or the closeted gay man who marries but fantasizes about men while having sex with his wife.
However, I"m going to agree with the previous poster and ask if it really matters what label you put onto it. Only you know who attracts you. That can be a valuable source of information. So your answer depends on what turns you on about transsexuals? Are you thinking of them as men or women or is it just knowing they're transsexuals that is a turn on?