I don’t understand why you cannot grasp that it is not the individuals they refuse to serve; it is the fact that they will not participate in the ritual that they believe is wrong.
Nobody is asking them to come to the wedding, just bake a cake.
This isn't a simple issue. On the one hand you have people's right to practice their religion as they see fit. On the other hand a person has the right to expect to be served regardless of who they are. But when you have a person's deeply held religious belief being labeled "discrimination" then I think that a person's religious beliefs should take priority, especially in the case of something as unnecessary as backed goods or photographs. But I also think they have an obligation to make it clear who they will serve and who they will not serve because of their religious beliefs. So I think they should clearly display their position, and people will buy their product or not buy it based on that position.