Your comment confuses me.

Masking helps prevent spread of illness.

Legality should follow morality, but the reverse often happens.

The grandparent comment didn't appear to suggest what you're clarifying question asserts.

That comment implied that discrimination was okay unless it was against a protected class. It is reasonable to ask what they would do if there was no such thing as a protected class. Would they approve of all forms of discrimination? Or would they do as your sibling comment did and decide that actually the concept of protected classes doesn’t matter and one can pick and choose which types of discrimination are moral/immoral.

It’s very easy to have a belief that businesses should be able to pick who their customers are or that they should be forced to serve everyone, but any opinion in between is very difficult to make logically consistent.

That's true, morality and legality are different, although one hopes laws will generally reflect society's view of morality. But you're right, discriminating against people based on their race or sexual orientation is immoral regardless of its legality. And requiring a mask in one's establishment during a respiratory pandemic is entirely reasonable, regardless of its legality.