His entire reason for disgust was either "trans people are icky" or "I'm not gay, but sleeping with a trans woman is a gay thing to do, which is bad."

Or maybe it's the fact that attraction is often more than skin deep? Factors like somebodies reproducibility, proneness to genetic diseases, and all sorts of things, can impact whether someone is attracted to them. These things are not obvious at the surface, and can only be known by asking.

As an extreme example - say you find the sexiest person you've ever seen, and you have sex with them a lot, yet, later you find out they are some highly unethical criminal who takes advantage of other people (but not you) - is it unreasonable for you to lose attraction to them? Of course not. Because factors other than how someone looks effect how we see them.

This is not a wild concept. I'm active in the LGBT community and several of my closest friends are trans, and even they can contend to this fact. Trans people who complain that someone lost attraction after they found out they are trans, are the ones with the issue - they've convinced themselves attraction is merely a surface matter, and that nothing else in ones life should matter. It's just not how reality is.