How is Support Gay Rights any different than Support Black Rights? I mean if we think he's "right" in a legal sense then sure, I guess, but if one refuses to make that cake I'm still free to think of them as a jerk from a moral/ethical perspective.
I'm sorry I'm wrong. I read to fast and thought it was about the customers not the specific message on the cake... rough day.... Anyway, sure, a bakery could refuse to make a cake with a message they oppose.