It refused to furnish a specific decoration for a specific kind of ceremony. This was not like walking into a place and purchasing a blank cake.
Based on the story it doesn't sound like it got that far. Once they found out it was a cake for a same sex wedding the store told them no, and told them why. Unfortunately they gave the lesbian couple the ammunition they needed, though at the time I'm sure they never in their wildest dreams thought they'd be hauled into court.