Apparently, the gay couple flipped the baker half the peace sign on their way out the door. For that alone, the baker should have been within his rights to refuse to bake them a cake. 

But I suppose flipping half the peace sign is protected free speech. As hands can be expressive, so too a cake can be expressive. The baker didn't want to use his hands to bake a cake that would express something he disagreed with. His hands should be as protected as their hands. 

That the SC took the case indicates they see a legitimate issue here to be resolved.