What if a group, for the lulz, orders a cake with a multitude of offensive messages? Like "white people burn in hell" and "the pope takes it up the ***" etc, etc. I guess that would be OK to refuse under hate speech laws, probably.
OK, now what if a satanic cult orders a cake with something that would offend most (all) religious people? What if just by making that cake the chef would feel really bad? What about a cake promoting paedophilia? Or Jihad? LOL, a Jihad cake, delicious.
For Pete's sake, you have to draw the line somewhere. A business should be allowed to trade under their own terms and conditions. It's not like that's the only cake shop they can order from. I'm sure there must be confectionery shops with gay owners, or a gay chef..