Anti-discrimination Laws are pretty much common sense when it comes to motels, restaurants, housing, car buying, and shopping for clothes, toys, etc where one has a business that is open to the public.
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The issue in this case is whether Masterpiece Bakery declined to sell already prepared bakery goods to a gay person because he was gay....or whether he declined to have his "artistic talents" "bought and sold" for a purpose that he thinks is offensive and wrong.
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No painter, photographer, cake decorator, or other "artist" can discriminate against anyone and refuse to sell items that are already made and available for purchase.
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The issue is whether The Government can force any "artist" to compose a KKK website homepage, paint a painting showing a man being lynched, videotape an execution, or decorate a "gay wedding cake" "just because a paying customer wants to hire him for his artistic talents." 
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Is the baker a "non-artist salesman" or is he an "artist"? The SCOTUS may decide.