Sam -- the difference between objecting to dating outside of your race versus objecting to dating outside of your gender identity is that -- by and large -- people of all racial/ethnic backgrounds copulate and share intercourse/intimacy in relatively the same manner.

For many cisgender people, sexual intimacy based on same/opposite genitalia is a prerequisite to the intimate component of relationships. I'd imagine some transgender people also feel the same way? (I can't comment directly, since I'm not transgender myself)

If people who identify as pansexual or omnisexual are sexually-satisfied beyond intimacy involving genitalia, that's fine for them. But they can't make that decision for everyone.