My mother was a caterer and in 30 years of business, I never saw a wedding cake with writing on it.
For “Symbolic Speech” to be protected, it’s meaning must be obvious and understood by the audience. I’m not sure anyone could make a case for that with a cake.
I believe that it is a better plan for us to simply let them know we are Christians by our example and love. Let them know you disapprove and then shower them with Christian love and generosity. You’d be surprised how much better example works compared to confrontation.
Even without words, the message is clear. I saw a cake similar to this at a "wedding" reception next to a real reception I was attending.
How nice of you. How Christian to endorse, through active participation in the ceremony.
Christians are called to be ‘separate’ and peculiar people. Participating in that way, one is not so ‘separate’.