This is all so easily fixable--except when someone wants to cause a merchant misery because he won't knuckle under to their demands. 
* Are there no gay-friendly bakers in town?
* Why is this couple illegally discriminating against gay and gay-friendly bakers by so steadfastly refusing to give any one of them the revenue? 
* Perhaps this baker could sell them a "non-wedding" cake they could get decorated elsewhere.
* The parties could shake hands and politely agree to disagree. 
* Baker offers a box of donuts to smooth out feelings. Or offers their next non-ceremonial order at $20 off. 
"Can't we all just get along?"