Deviation from the norm is always seen as dangerous for a lot of people. The average man usually is able and will adapt to change but some of them don't.
Scarecrow tactics in politics. "Gay and trans people kill the christian family values", " Gay people mindwash our children" (reference: my country, aka Hungary)
Others (For example a friend of a friend was at a party, met with a girl and she later on spilled the beans just before the sex act. She was trans and that guy gut furious. Idk him probably he was already a transphobe before)
There some reasons:
EDIT: Sry didn't read the title