A "Devout Christian" caterer should not be forced by law to cater a gay wedding any more than a gay caterer should be forced to cater a reception in the basement of the Westboro Baptist Church. ( Or whatever the name of that nasty bigoted anti gay church is.)
Unless you argue in favor of the latter, you're full of shit to argue the former.
Or how about forcing a Black caterer to serve a KKK rally? Not quite as analogous, but not too far off.