You are missing their point.

Consider a cis woman, and compare her to a trans woman who is has been on hormones their entire adult life (never went through male puberty) and has had the complete series of surgeries to get her genitals and secondary sexual characteristics perfectly in line with her gender.

Now consider two more women, one of which is similarly cis, and the other is similarly trans, but you don't get to know which is which beforehand. Would your attraction magically know which woman had female genes? Of course not.

So the point is if it is the knowledge of their being trans is the entire reason for your lack of attraction then that is a negative bias against trans people, commonly referred to as transphobia. If you can base your lack of attraction on character, behavior, physical features, compatibility, or even just what sex organs they have, then that is not transphobia.

Whether you like it or not, that is a prejudice towards trans people. Call it a bias or a preference or whatever else you like, but its there. You're allowed to date whoever you want for whatever reason, but those are still prejudices towards or against particular traits about someone.