The baker in question specializes in weddings. They may refuse customers if the function at issue is not a wedding. The baker considers the formalized celebratory union of anyone other than a man + woman to be not a wedding.
Now, based on no legislation (as one dissenting Justice makes scathingly clear), a new "protected class" has been created. The baker still considers a pair entering that "protected class" not a "wedding", a view the ruling notes still should be respected and free to hold & act on.
Well, the SCOTUS just decided that same sex unions are, in fact, weddings and sex is an existing protected class based on actual legislation and precedent. That means the baker would be discriminating illegally as a public accommodation on a federal level if they refused their service based on the sex of that event's participants.
Non-existent because they do not provide a public accommodation and are exempt from legal entanglements based on discrimination.
IOW, even though they provide a service, it is not the same class of service, legally, as a cake maker or any other for profit entity serving the public.
Now, based on no legislation (as one dissenting Justice makes scathingly clear), a new "protected class" has been created. The baker still considers a pair entering that "protected class" not a "wedding", a view the ruling notes still should be respected and free to hold & act on.