It's got nothing to do with expression, opinion, belief, or freedom of speech.
If you want to run a business, you must run it by the rules. If you don't want to obey the rules, you have the option not to run that particular business.
That's it. That's all of it. Literally everything else is just a smokescreen to try to obscure this simple fact.
The US government says businesses (not individuals, or clubs, or groups, or wedding designers, or bakers) cannot discriminate against customers on the basis of their membership of certain specified classes.
They are right to do so, and the constitution in no way prevents them from doing so.
You can be as bigoted as you like. But you may not exhibit that bigotry during any business transaction.
What's difficult about this?
It isn't difficult. Why on earth do so many people think when they're disagreed with it's because people find their reasoning "difficult"? Your reasoning is tediously easy to follow. The reason your argument gets rejected is because it's stupid, evil, and illegal, not because it's "difficult".
Selling labor to an employer is a business transaction. If you may not exhibit your bigotry during any business transaction, then when your boss quits and your metaboss hires a woman to replace him, but you're a fundamentalist who takes St. Paul's idiotic "No women bossing men" rule seriously, so you quit your job and go looking for another man to sell your labor to, the government can prosecute you for quitting your job. That's a Thirteenth Amendment violation.
Now I know what you're going to say. You're going to say buying labor from an employee is a business transaction but selling labor to an employer is not a business transaction, and if I can't see this it's because your argument was too "difficult" for me, not because
they are one and the same transaction. A transaction is not magically both a business transaction and not a business transaction because reasons.
If you want to run a business, you must run it by the rules. If you don't want to obey the rules, you have the option not to run that particular business.
That's it. That's all of it. Literally everything else is just a smokescreen to try to obscure this simple fact.
What is or isn't "a business" is not a matter of "simple fact". It's a matter of
arbitrary subjective labeling. If your argument were sound legal reasoning then the government could cancel any constitutional right it felt like whenever it pleased by the trivial expedient of labeling whatever activity it's suppressing "a business". "Business" is not defined in the Constitution. But that's okay -- we don't need a definition of it, because "unless you're running a business" is featured in our Bill of Rights exactly as prominently as "some animals are more equal than others".
What is a "simple fact" is that freedom of speech is a
constitutional right, and nondiscrimination is a
statutory right. Arguments like yours are transparent attempts to rationalize making statutory law trump constitutional law because the arguer likes the statute better than the Constitution.