I didn't even mention sex, yet you automatically linked trans people to genitalia.
It's almost like relationships are often built on sexual attraction. That's how it goes in the animal kingdom.
As for other stuff you said: some people want to conceive children themselves with their partner. While it is possible for transwomen to get pregnant with donor eggs, it is not with their own egg (because they were originally male, y'know). That might be a deal breaker for some. Same with transmales, they can't produce sperm so donor sperm is needed.
Rejection at such a late stage might be on religious grounds as well, which is why I brought it up. Even if they've gone through a full gender reassignment, transsexuals are considered to be the sex assigned at birth in these religions. That means a man can't marry a transwoman, it's considered gay marriage.
We know damn well that's not the same thing.
Only dating someone who's a part of your religion is different than not dating an entire race. It's similar to the issue above: Saying "I don't think asian men are attractive" is claiming all Asian men have the same shape, same colour, and same size and is bottom line, racist. Someone from Thailand doesn't look like someone from Korea, or someone from Cambodia or India or China or Malaysia. Dating people within your religion is not looking down on people for a stereotypical look they have.
My point on religion was mostly in regards to transsexual people, so I don't see why both you and bellionaire decided to focus on race there. Transsexuals are seen as their sex assigned at birth, even if they go through a full gender reassignment surgery. That means that a male and a trans-girl marrying is not allowed.
Though since you both mentioned race, Muslims of certain movements are encouraged to only marry within their "race". For example, in these movements, white and Asian people are considered kafirs and can never hold the same rights as a Muslim of that certain "race", even if they fully convert to islam. Thus, they are rejected based on race.
I don't agree with it either, but I definitely don't agree with bullying these people for their decision.