When you sign a business license application, you agree to abide btpy state laws. The baker knowingly broke the law. There are bible verses about that. 
Like most people, you do not understand the word “unique.”

You can argue art all you want, but it is a cake. His purpose is to make money. 
He could make a custom cake for a wedding between Kim and Kim.
How does he know if it is for a gay wedding?
He would have to ask. 
What is the difference? He makes a wedding cake and does not k ow if it is going to a gay wedding or a straight wedding.

How can he be offended by that if he dows not know?
The answer is the baker is being an immature baby that does not want to make cakes for a protected class of people. 
He is a bad Christian, he went back on his word. He is choosing to be offended over something that is not his business. 
His “art” can be recreated by any number of bakers. His are not special and certainly not unique. Religion is not an excuse to discriminate.