Well this is a thought provoking CMV, so thanks for that. I'm going to adopt an unusual approach, focusing here:
What if as an Atheist you were forced by law to go photograph a jesus convention and were not allowed by law to say no?
The law does not force you to do anything. It can and does apply coercive pressure. But that is not the same thing.
For example, I have seen--with my own eyes--brazen employment discrimination against blacks. As recently as 2010. This shit has been illegal since the Civil Rights act of 1964, and yet it still happens to this day. Not often, not everywhere, but if you are in the right pocket of the deep south, the civil war never happened.
What laws like the Civil Rights Act practically do, is let people sue if they face discrimination. But let's talk reality. In 2015 it is probably easier just to find a non-racist employer, than to invest time and energy into a lawsuit. Second, suing racists just produces poorer racists, it doesn't fix racism, so that is not attractive, and racists are poor to begin with, so you are not going to get any money. Third, discrimination is hard to prove.
The net result of all this is that if you are a racist employer in 2015, you can get away with it. Sure, you can't grow to Walmart size, but you can skate by.
What, then, is the role of the law, if not to end discrimination? The role of the law is to say what is right and what is wrong. Without the law, a racist could say that they are just a minority view, ostracized by a delusional society. With the law, they must say that they are a criminal, that they have turned their backs on the democratic process, that they reject the system of government, the values of justice and fairness that even other racist white men fought and died for. There may be some principles that are worth becoming a criminal for, but I don't think even most racists believe that racism is among them.
Back to gay birthday cakes, no law really forces people to bake them. If you are really uncomfortable then you are not going to do it, and to hell with the law. That is the reality.
What the law does is it says what is the right thing and what is the wrong thing to do. Providing personalized service to everyone is the right thing to do. It is not always going to happen, and people are going to exempt themselves out of it, but that is a different issue.
An analogy can be made here to the KKK. They are a horrible, deplorable, hateful organization. They make me very uncomfortable. But there is no right for me to feel comfortable. If they want to march in my town, shop in my store, print their literature in my print shop... I know what the right thing to do is.
I'm not saying I'd actually do it, mind. I'm human, and I don't know if I'm actually capable of stopping myself from spitting in their food. But in my heart of hearts, I know the right thing to do.