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’s not a slur, but I think the way you worded your joke might have come off in a way that made her feel like you were using it as a slur.
Oops… Makes sensde