There's lots that's been written about the roots of homophobia and transphobia. _Whipping Girl_ is one example, which I already mentioned. It's not really all that weird. Racism seems weird when you ask "why would people hate each other based on the color of their skin" -- not as weird when you think about othering and how according to the most popular formulations of power, one person having power means the other person has to lack it.

(This may be why cis people are so obsessed with the idea that you can always tell who's trans by looking at them, BTW. If people in the out-group can't be identified easily, then society loses the valuable benefits of having an out-group to marginalize.)