Religion is a good example of one... people can and do change religions, but we still shouldn't discriminate based on it. There are other examples as well of things that people can change that shouldn't be discriminated based on.
If someone thinks black people are innately inferior to other races, it's culturally acceptable for me to discriminate against them, but if their god tells them that black people are innately inferior to other races, I'm not? Heavily suspect in my opinion.
But also, discriminating against things people can control is really just one party trying to apply soft power over another, and I am generally against it. You might be able to legally say you can't work at this company if you like to go bowling in your free time, but I don't think it would be a right thing to do.
For some jobs, other it may not matter.
In fact, US courts have upheld explicit discrimination against people of higher IQ: https://abcnews.go.com/US/court-oks-barring-high-iqs-cops/st...
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.
If push came to shove I'd remove religion from any list of protected classes gladly.
Also, political belief is highly predictable based on demographic information such as geography, race, religion, and sex.