Both religion and national identity are changeable, they are 'just opinions', but both are key historical examples of protected class.

You can say that both are predicted by geography, but must conflicts happen in situations where populations of different identities are mixed in one geography. Then you can say it can be predicted by parents' opinions, but that is true for many other opinions, including political ones.

In some sense, protection of changeable characteristics is more important than protections of non-changeable characteristics, because bullying based on changeable characteristics can be effective to pressure conversions of victims.

And it is question how political opinion is just an expression of deep personal characteristics that are not much changeable and perhaps even partially given by genetics.