But this issue is dependent on who the person submitting the order is. It all changes when you put that content up for sale. At that point it's no longer an issue of creation, but of commerce and thus a denial of service. The artist can for the most part sell who he wants to, but he can't use race, creed, origin as criteria for it. And once he admits he is indeed discriminating, you have a hell of a problem on your hands.
I really don't think it is. You seem to be saying that his denial of service is based on the fact that they men are gay? But he would make them a birthday cake, no? So, his denial is based on the content of the cake and not on the fact that they are homosexuals.
If he said "I don't make cakes for gay people," I would be right there with you.
I don't think that this talk of protected classes applies, because we are talking about content and not people. The government can't, at the risk of repeating myself, discriminate between content that a creator has to be forced to provide on demand, and content that does not receive that benefit.
Should the state require a printer to print an pro-LGBT leaflet? If the person submitting the job is gay, would refusing it be an example of discrimination based on a protected class if the printer refuses? Or is it just the right to refuse certain types of content.
What if it is an anti-LGBT leaflet? Should the state force a gay printer to do that job?
If you say yes to the first one, but not the second, you are probably just favoring the content you agree with and not resting your decision on any universally applicable principle.