their existence proves that sex isn’t just a binary.

This argument has always struck me as odd as in virtually every other discussion we would accept that the exception ‘proves the rule’.

Humans have two hands, except when they don’t due to something impacting fetal development.

Humans have two kidneys, except when they don’t due to an error in fetal development or as a result of disease or injury.

There’s diversity to sex that requires a more complicated scheme to account for everybody.

Or just let the exceptions be exceptions with no social stigma rather than refusing to recognise that the vast majority of humans, and mammals, can be accurately identified as one of two distinct sexes.