But you’re not forced to serve them items not on your menu. If the government forced you to prepare collard greens against your will, that would be involuntary servitude.
Probably not, but I see where you are going with this. The government can easily issue regulations that all restaurants must serve x.
The government would have to have a reason to mandate that all restaurants provide collard greens or a court would probably throw it out. But I could see the government mandating that all restaurants must serve water as a healthy alternative, and a restaurant that didn't historically serve water as an option would have to comply. And the courts would support it. They could do the same thing with Brawndo (idiocracy movie ref), if you can make a compelling case.
Wedding cakes are on his menu. Wedding cakes with 2 same sex figures are not. Not sure if they asked for 2 same sex figures or not. It may come down to whether the court feels he's really participating in the wedding or just selling a wedding cake. And/or whether the cake was any different than the cakes he normally sells.
Even so, some of the analogies earlier in the thread are appropriate. Could you make a black person sew KKK outfits.