We are not born with an attraction or sexual interest in specific things, just a general, vague interest in “maleness” and/or “femaleness” (or neither) and our more specific interests are formed around what we learn that means, and our experiences. Androphillic people are not inherently attracted to dicks, gynephillic people are not inherently attracted to vaginas (please let me know if these are problematic terms or if they’re trans exclusionary).

So yes, it is transphobic not to date a trans person because of their genitals.

For instance, I am a cis gay man. I am into dick. A lot. And yes, I am not a huge fan of vaginas. But you know what? That’s not something I was born with, it was a result of the transphobic circumstances surrounding my sexual development.

As a result, I am transphobic. And I know that. Yes, it is essentially “just a preference”, but it is a transphobic preference, and you can’t use “preference” as a defense.

Acknowledge the transphobia that shaped your development. Grow from it. Use that knowledge to educate other cis people and prevent your ingrained transphobia from hurting others. And try to use it to make the world a safer place for trans people.