The above does not make sense in the context of your following sentence:
The only difference is that they plan to use the cake in a same-sex wedding.
If it is all the same, how could it possibly be known it is for a same sex wedding?
Because the two people getting "married" may come in to shop for the cake together. Couples often do.
“Because the two people getting “married” may come in to shop for the cake together. Couples often do.”
How do the shop owners know they are not bothers buying a cake for their sister’s wedding?