It's certain cis people being angry that trans people exist. In their imagination, they live in a cis-only bubble, and every time they're reminded that trans people exist, it threatens their view of the world.

Specific to sexual orientation: They declare that the One True version of their sexual orientation (gay or straight) is only for cis people by definition. Then they act as though trans people are on a mission to destroy cis people's concept of sexual orientation.