This is going to sound stupid, but one day I was talking with my ex-friend "Carol". While we were friends, someone said the word "heterozygous". Carol mentioned that she was straight, so I jokingly said "hetero, like you, Carol!'

She acted as if I called her a slur and got very offended, saying "How dare you call me 'hetero'?!" so I apologized.

(We are ex-friends because of drama [not this], which is kind of a long story, but I guess not the point of the post.)

I have heard many straight people call themselves "hetero", though, including myself when I thought I was straight.