I suppose you could discriminate on the basis of dress code and similar considerations.
Should it be legal to require that the receptionist at your law firm wear business formal attire, and fire them if they refuse?
Should it be legal to insist on natural hair colors, no visible piercings other than earlobes, and no visible tattoos in the workplace? Should it be legal to fire anyone who doesn't comply?
Should it be legal to require that everyone dress according to the gender they present as*, in the workplace? I.e., if your boss thinks you look like a dude, you have to wear pants, and if you boss thinks you look like a lady, you have to wear skirts and dresses?
Should it be legal to require employees to answer to the pronouns others perceive as correct for them, and fire them if they insist on other pronouns?
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*As which they present.