Good question. I think what it comes down to is that the idea of someone being trans is just kind of foreign to me. I never met someone in person who was trans until I was close to 40, as far as I know. So for most of my life I categorized people, at least as far as attractiveness and dating goes, without distinguishing between sex at birth and gender identity.
So while I treat (or hope that I treat) trans people as appropriate for their chosen gender, it doesn't come completely naturally to me. It's hard for me not to think of a trans woman as "a man who wants to be treated as a woman", even though I know that's not what they want. And while in day to day interactions I can just ignore that difficulty and treat a trans woman as a woman, when it comes to romantic interest it is not so easily ignored.