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