OK, so what if instead of a message, the customers asked for them to paint the cake with Ernie and Bert fucking, and their muppet cocks flopping about? Would the bakery still have to fulfill that order? Because the customers are asking for a cake, after all. Nothing is being asked differently from them, either, it's just a cake.
I feel like the motif that is being portrayed is important. If I sell a product that requires some sort of creative process, and a customer wants me to create that product in a shape that I don't want to create, why can the law tell me that there's absolutely no circumstance for me to refuse creating this product? What happened to free choice? Why would I have to be forced into doing that - if don't wanna bake a cake with Hitler dancing in swastika-shaped confetti, or make pottery in the shape of a horse dick? I feel like it's up to the business owner to decide which product they want to make or not.