Mike and Bob are customers, a bakery bakes cakes. If they want to buy a cake they shouldnโt be turned away because of who they are. The bakery makes food, where the food goes and what someone does with it once it leaves their premises is none of their business.
They can certainly refuse to let Mike and Bob to get married IN the bakery, and they donโt need to make any type of pastry that they donโt offer on their menu, but a wedding cake is a wedding cake. So in this case they would be making an exception to their normal business practices (making wedding cakes) because of who wanted to buy the cake. If they sell one to Mike and Roberta, they should sell one to Mike and Bob or to Michelle and Roberta.