Oh bullshit. What if an atheist walks into the store and buys a cake and wants the atheist symbol embellished on it. That could violate the beliefs of the store owner as well. Except, currently you cannot discriminate against people based upon religions (of which absence of one is protected under law as well).
Like literally the worst argument. If it was about artistic expression; Republicans could discriminate against Democrats. Democrats against Republicans. Burnettes against Blondes, Blue Eyed people against Brown eyed people. It's just a dumb argument Hilltopper, sorry bud.
For your Nazi Group Jewish Bakery denying the holocaust; they would also be discriminating and it would be wrong. Look, people are SUPPOSED to be protected equally under the law period. No matter how vial their speech, or how vial their actions or beliefs, so long as they don't impede/harm others directly. That's why the KKK can still hold rallies, thats why the wackos of the Westboro Baptist Church can go protest military funerals. Thats because we live in a country that is run by a rule of law. Our country isn't supposed to take sides; it's supposed to make a level playing field. If you don't want to play in a level playing field, because of the chance someone might walk into your store and ask you to perform a service that you offer to everyone else, but something you don't agree with, then don't open your business.
Also, it's poor business. Money is money, no matter who gives it to you.