The protected class argument has two points:
1. A characteristic of a citizen was used to trump those who claimed their property rights were sufficient to support their racism. Thus property rights are not supreme when society views an absolutist position as detrimental to society and fundamentally evil
2. Exercising the right of self-defense is inherent property of being a human being. Being of a protected class is also inherent in you. Your rights to function in society were not open to property owners (who opened for business and expect tax payer paid for services) to use as discriminatory decision points. Thus, this inherent property of self-defense (like your race, etc.) should not be open to discrimination. I see no reason to allow discrimination based on race and allow it because you are a human being with the right of self-defense (except in the techy case, I mentioned earlier).
Protected classes are a social construct - we still argue as to what that protection encompasses. Clearly some societies deny those rights still. In Saudi Arabia, women are not allowed to drive. Is not freedom of travel something to be granted to all? I regard self-defense as inherent and not a grounds for discrimination when you open yourself to the public for business.
You don't have to have a business. You can be a wage-slave or a hunter-gatherer.
Who says you have right to a business, expect tax payer supplies services and then want to deny rights to people?