If he puts a bride and a groom on the cake, he must put a groom and a groom or a bride and a bride on it.
The law says to treat people in protected classes the same.

Every person in America is a member of at least three protected classes. 

The baker has made many custom cakes for gay weddings, he just did not know. He is only offended when he knows it is for a gay wedding. He is acting like a child.