It doesn't.

"I refuse to do business with you because I do not support your beliefs or practices" doesn't work when the government has recognized your beliefs and practices as protected. Your watercolor/monochrome discussion is false; if your trade is in painting watercolor, no one can force you to paint in tempera. However, if your trade is in painting klu klux klan rallies in watercolor, you have a civil case that you can't refuse to paint the Million Man March in watercolor.

You should be equally disallowed to refuse a cake with "Jesus saves" on it. You're putting your prejudices ahead of your community, pure and simple. Something lost in all this is that it's a rare person who wishes to do business with someone that hates them. I think it's safe to say that in every case where your trade needs to be protected by law, you're dealing with a situation in which both parties wish there were more choice. Sometimes that choice isn't available.

NO ONE benefits from you standing on your principles and refusing to put whatever the hell your customers want on their cakes. It's a fuckin' cake. But if it means that much to you, that means that you need to be compelled to decorate their fuckin' cake because society is about putting up with other people's fuckin' cakes. That's really what this is about: "religious freedom" as used by the right means the "freedom" to deny people their place in society because religion.