Why? In our society we generally do not force private citizens to say things they don't want to say.
What is so special about cakes?
Nothing's special about cakes.
What's special is the discrimination.
Is that your problem with this whole issue? The actual 'issue' went over your head?
If you have a law firm, you can specialize in real estate law and turn down DUI cases. But if you are a DUI firm, you can't refuse to defend gays, blacks or idiots in DUI cases because they're icky.
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We are not talking about discriminating against people but messages on cakes.
Exactly how do you discriminate against messages?
Do messages have civil rights to free speech?