If you are interested in a person you assume to be cis, and they have had bottom surgery so the only real way to tell if they are trans is if they tell you, and finding that out makes you decide against having sex with them, you're a transphobe.
If you are interested in a person who is trans but has not had bottom surgery, and that makes you not want to have sex with them because you are not attracted to penises/vaginas/etc., you are not a transphobic
jesus I don't even know if I want to get into all this
Please keep in mind that prejudiced "preferences" in dating largely revolve around race in the United States, so it's not limited to the experiences of trans people. For example, Asian men are frequently discriminated against for supposed effeminacy, and black women are discriminated against for ... too many things.
I am tempted to approach the subject of trans people not getting fair dating action in a similar way. There's not much of a reason to pursue a relationship with prejudiced/uninterested people. It sucks and it's bullshit, though, that minorities of one stripe or another are expected to be the understanding and accommodating people when they're the ones being treated unfairly.