"In fact, he is being asked to cater the wedding and to write something on the cake that he finds offensive (presuming he finds those prohibited actions offensive). "

Source please.   I have never read anything that he was being asked to write anything, nor that he was asked to cater the wedding.  And the "decorations" he is referring to is traditional wedding cake decorations.   This couple was not asking him to make a cake with 2 men kissing on it or whatever, it was just traditional wedding style decorations.   Decorations to where if you looked at the cake, you would say "yep, that's a wedding cake".

The fact of the matter is, this guy didn't want to make a wedding cake for a gay couple.  At all.  Nothing to do with anything else and there is nothing else this couple could have asked for that would have convinced this baker to make them a cake for their wedding.