Many societies and cultures allow for forms of changing your gender, or have other gender roles that fulfil these.

The concept of “mental disorder”, just like “gender” is a social construct.

In a society where these other genders or changing genders is not commonly allowed, like modern western cultures, someone who doesn’t fit into the binary vision of gender might be seen as having a “disorder”.

Just like back in the day, in a society where gayness is erased and does not exist, a gay person might be seen as having a mental disorder.

It’s very important to note that what is considered a “disorder” or not is very much a social construct.