RE: He was refusing to let his artwork honor a particular event.
Bingo, THAT is the main issue. The Baker could have served the gay couple or any gay who came in for ANY cake ( e.g. a birthday cake ).
Clearly a FIRST AMENDMENT issue which SHOULD protect the baker.
If he refused to make a cake for the (statistically unlikely) same sex marriage of two heterosexual men (for tax reasons perhaps) would this even end up in court?