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.
It absolutely isn’t but that doesn’t stop certain idiots from pretending like it is. Same with cis people who don’t like being called “cis” because they want to believe they’re the default and any language that identifies them as otherwise is offensive to them.