So if a kosher shop owner isn't obligated why would a Christian bakery be? I am not getting it. Lots of restaurants reserve the right to refuse service. Nothing new. Cakes, and printing require people to write things contradictory to their faith. It goes as far as forcing a pastor to perform a wedding. Wrong in every way.
Businesses cannot be compelled to sell what they do not carry. The example of the Kosher deli fits that, for a Kosher deli would sell anything on their menu to anyone that visits. So if a neo-Nazi wants a hot pastrami on rye... he will get a hot pastrami on rye. But if he wants a bacon sandwich, he won't get it, as the deli doesn't have that on the menu.
The case of a Christian bakery that sells wedding cakes does not fit that criteria. Anyone that walks in has the expectation that, since it is on their list of items to be sold, that they can purchase a wedding cake. If the bakery advertises that they can put any message/image on the cake, then they are saying that they *WILL* put *ANY* message/image on the cake. (barring illegal messages/images, if any)
Now if the baker advertised that he only had chocolate, vanilla, pinapple, and fudge flavors... and a customer demanded a peach flavored cake; then the baker can refuse - as it's not an item they sell.