That's exactly my original argument.

"I determine you're ugly but you can still come into my nightclub" is not that much better than "I determine you're ugly and you can't come into my nightclub"

How about “I’m not allowed to use attractiveness to determine if you can come into my nightclub”

Good luck ever enforcing that.

Because the above counter-argument is that you only belong to this class when people put you in it.

People aren't in or out of a protected class. Protections for race, sex, orientation, etc apply to everyone, even straight white men. Protections for religion even apply to atheists.

The laws prohibit certain behaviors, like excluding people from nightclubs on the basis of skin color. They could easily also prohibit excluding people from nightclubs on other appearance criteria.

Of course no one can control thoughts, and some people will still feel and think hateful things, but that's true for every protected class.

Which is why rather than protected classes there must be protected characteristics.