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
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Ye, can't imagine leaving someone I love for not being able to have kids.
Personally I'd think being upfront with one's motives would be enough - like if I were in the position of hooking up with a trans cutie & actually cared about having kids (I don't) I'd probably suggest something along the lines of: "hey its really important to me to have kids some day with my own genes because [why?] - so if you're into it I'm down to hook up, just don't want to get into a longterm commitment."
Not sure if that's a good way to go about it or not but that would be my instinct