Alas, that's not our legal environment.
So the question is to what extent government compels service to whom.
Compelled speech...should I be forced to bake a cake which says...
"Atheists will rot in hell"?
I don't know where the line will eventually be drawn.
Personally, I think the line that should be drawn is one making a more useful distinction between what is and is not considered public accommodation.
In my view, the use of establishments that provide goods and services in a unique and creative manner (bakeries, tattoo parlors, sign makers, etc...) shouldn't be thought of as public accommodation. Owners of such institutions should have veto power over what they will or will not create, regardless of who the client may be.
A Christian baker should be able to refuse to create a cake for a same sex wedding.
A gay baker should be able to refuse to create a cake for a Catholic wedding.
A vegan sign maker should be able to refuse to create a sign for a meatpacker's convention.
A Muslim tattoo artist should be able to refuse to create a "Muhammad wearing a bomb in his turban" tattoo for a Danish cartoonist's forearm.
An atheist cake maker should be able to refuse to create an "Atheists will rot in hell" cake.
The client should take their business elsewhere. Maybe write a bad yelp review. But the owner should NOT be forced to create something he or she refuses to create, and the owner should NOT be sued for discrimination.