How is my scenario irrelevant? He is proud if it goes to a heterosexual wedding but disgraced it the same thing goes to a gay wedding? 

Why would he take less pride in his “work of art, custom cake” if all of a sudden it is used in a gay, satananic wedding?
The baker is choosing to be offended. 

Is the baker going to ask people if they are gay before agreeing to make a custom cake?
He is being a petty child.

So what if you think the cake is art? I am not going to argue that with you. 
Do you think artists are allowed to discriminate?

If an artist sells to the general public, the artist can not discriminate against protected classes.
If the baker wins, it will be legal to discriminate based on religion. I guarantee you black people will be denied service somewhere on religious grounds. 

You can make all the semantic arguments you want. The baker discriminated, no one is arguing that. 
He offers custom cakes to divorcees and adulterers, but not to homosexuals. He is a hypocrite.