Religion does not confer immunity; human sacrifice is still frowned upon.
In my view, questions of religious expression or freedom of speech are distractions. The only amendment relevant to the discussion is the 13th.
Party A wishes to have Party B perform an act that Party B is not willing to perform. That is the definition of coercion. There are reasons for which coercion is justified; to settle this case we have to decide if to get a wedding cake is one of those reasons. (Or, more grandly: to induce people to perform gestures consistent with mainstream values when they do not hold them, or else fabricate socially acceptable excuses, where concerns of reputation and future profits are not sufficient inducement.)