Arguing for legal discrimination is weird.
Arguing the "slippery slope" that will happen if the baker is not successful is even weirder.
If the baker is successful anyone will be able to discriminate for any reason and just chalk it up to religion.
A doctor could refuse to give you emergency medical treatment and claim it would be against his religion. 

Again, the baker agreed in writing to abide by Colorado law. He was in flagrant violation of the law.