I am not missing anything.
“An artist can pick and choose who commissions him to perform his work.”

Since when can an artist discriminate against a protected class? Could you give me a source on that?

You must have missed my point. You are acting like Jack Philips was asked to make some sort of offensive cake or design. He was not, he was asked to make a wedding cake just like everyone else wants. There was nothing different abiut it. Again, if the gay couple did not order it themselves, he would nit have known it was for a gay wedding and made the cake. 

Jack Philips refused to do exactly what he has done hundreds, maybe thousands of times before.
They are not asking him to make anything special, just a wedding cake. He wants to be able to say no because it is a wedding cake for people that are gay.