It's not really that surprising. Just as there are straight men who are misogynist there are people who are attracted to femboys, trans people, and/or other gender nonconforming people who hold prejudices against the same group. It's massively oversimplifying human psychology to think it just comes down to a single linear axis of like/dislike. For some people, it might be that simple, but it's not always the case.
Take the song Hellfire and replace Esmeralda with a femboy, everything else can stay the same.
Instead of simply accepting that one might be attracted to a spectrum of different kinds of people, they immediately feel internal guilt, directed outwards as hatred, because it might give them some sort of label they've been trained to despise.