Yup, Free Will is the answer. But “Adam & Steve” wanting a wedding cake from a particular place? The owner DOES have the right to refuse service I suppose, but to really PO the courts and the “O” administration, he should just issue 2 brides and 2 grooms in a ziploc baggy, and let the “happy couple” decorate their own cake with the final touch 
That way he can just say “I never know about the couple, it is a choice they make how the cake looks”
Do not offer writing on the cakes, or feminize one of the names and make the other masculine.
“OOPS, I thought I heard you say ADAM and EVE, not Steve!” My Bad!
Here’s a can of writing icing, fit it yourself! I’M TOO BUSY!!!
Beat them at their own game
"ROGUE ELECTRICIAN" Hoping to be around to re-energize the New World.....
Cogito, ergo armatus sum