In fact, by law, all contracts are VOLUNTARY, and if shown to be entered into by force or coercion are, if challenged, avoided(legally cancelled) by courts.
You cannot force someone into a contract.
Asking a baker to provide a cake for (whatever, but let's say your big fat gay wedding,) is asking him or her to enter into a contract with you. ( a verbal contract, but verbal contracts are enforceable, assuming they meet the other criteria required of contracts)
The baker, wearing his klan hood or not, declines to enter into the contract. He gives no reason for his decision. (call this part - Don't ask, Don't tell)
I see no reason for the government to get involved. Simple freedom of association. I choose to NOT enter into a contract, and I don't have to give you a reason. In fact, engaging someone further, and giving reasons is counterproductive and opens the door for excuses to litigate based on illegal discrimination. Remember, discrimination is not always illegal.