So...where is the line?
Is this the ONLY wedding-cake store in a reasonable distance from this couple? Why not vote with their wallets and head two streets over? Would YOU want a cake someone was forced by a court to make for you? (I know I wouldn't -- no matter how good it was it would taste bitter to me.) Granted, I do NOT agree with her, but she was honest with them about WHY -- and it's a small business, she's the owner and she is NOT telling anyone else to refuse service to them.
I do have a small sign that says "I reserve the right to refuse service to anyone for any reason or for no reason".
When can I refuse to sell someone a massage or to sell them something at a vending table? I've fired massage clients and told potential customers to take it elsewhere -- and seriously, I'm now wondering when I'm going to get a summons because "My client couldn't help swearing at you and trashing your product, s/he's had a rough life and you should be more understanding, s/he has psychological problems." Or "You had no right to refuse service to my client just because s/he grabbed your ass, you're a masseuse and you should expect that."