A Baker should be able to refuse to bake a cake he doesn’t want to make. He shouldn’t even have to give a reason. Anything less than that is by definition forced labor.

“If that Christian customer instead asks for a blank cake that they’ll decorate themselves, the baker must sell it to them or else they are violating the equal protections clause.”

This is an issue too though. The only person who can enforce the requirement that the Muslim Baker sell the cake is the government and the only way the government can force someone to work is through force. What you end up with is the government using threat of force to require someone to work. Which is slavery at its core. Anyone should have the right to refuse work if they don’t want to.