You stated: "The cake baker did draw a line at the customizations that were requested."  There is one problem with that argument There were no customizations requested.  The couple sits down at the wedding cake table, and he goes "I don't do gay weddings."  There was absolutely no discussion on customizations.

And if ordering a wedding cake is "customizations", then he was willing to do those exact same customizations for a straight couple.  The only difference is that the customer for one cake is straight, and the customer for another cake is gay.