I think there is a very disingenuous omission regarding some of the more pertinent details regarding the reasons for not supplying a cake and it has to do with freedom of expression.
In this case, unlike many of the other cases regarding cakes and flowers, the owner of the store was willing to sell any cake from an available catalog.  The rub comes from the fact that the couple wanted a unique special creation that would not be found in any book, manual, or recipe.  The courts have ruled (read Hands On Original from Kentucky for more info) that when creating something original is an expressive act.  And because of that, there must be a higher threshold met in order to force an individual to express solidarity to an idea that the creator does not agree with.
Here is a great line from Slate:
"Hands On Originals was not refusing to sell the very same items to LGBTQ individuals or same-sex couples that it sells to other customers; it was refusing to sell a particular design."