These people were not denied service. They were not kicked out of the establishment. If they asked for a birthday cake, a retirement cake, or a dozen sweet rolls to take into work for their fellow employees they would have gotten them.
If one of their heterosexual friends came in to purchase the cake for the homosexual couple, they would have been denied as well. The sexual orientation of the customer does not matter - it is only the event.
Events do not have rights, only citizens.