What if one man had shown up asking for a wedding cake for John and Kim, what is the problem? 
Maybe Kim is a man, maybe Kim is a woman. The baker would not know if he was not told. Would the baker refuse to make the cake because maybe Kim is a man? What does it matter? 

What if a very effeminate man pranced in and asked for a wedding cake. What if the baker refuses and the man is straight?

This baker is a terrible Christian, a terrible man and from the looks of his dirty shop, goatee and no hair net, he makes hairy cakes.