I do not understand why there is a difference in how ethnicity and sex are considered and would welcome any (constructive) responses to shed light on this.
People don't seem to go in for changing their ethnic identity (unless perhaps they learn something about their ancestors that might change this). People who change how they identify in terms ethnicity appear to be met with considerable negativity and this is viewed as unacceptable.
Can you change how you identify in terms of ethnicity and should someone be able to access service for a specific ethnic group if that is how they choose to identify?
How does being a 'race faker' differ from being transgender?
https://metro.co.uk/2023/02/21/race-faker-called-out-by-her-mum-who-says-she-is-as-white-as-snow-18320358/