Again, the message excuse seems to be a relatively new reason for refusal, arrived at after consultation with a lawyer. Effectively they are offering a lesser service to gay people, I.e. Not wedding cakes.
not they're not - they haven't refused to sell a wedding cake, they've refused to sell a wedding cake with a particular message on it
there is no indication that the customer in question couldn't have otherwise gone into the store and been able to buy any cake anyone else could buy