From my own perspective, it's being on the receiving end of a cock that is the problem.
In my mind, the act of taking cock is gay, be it from a man or a trans woman. At the same time, it's okay to fuck a trans woman because they are feminine and identify as a woman (somehow that isn't gay in my mind).
Not saying it makes sense (it doesn't) but that's how my (relatively) straight mind works. Give dick okay, take dick is bad. Which to a trans woman must seem incredibly unfair. That's why I would never get in a relationship with one, as I'd feel selfish for ignoring their cock.
If I was more comfortable with my sexuality it really wouldn't matter one way or they other. I wish I thought that way. Life would be simpler and more fun