Tell them that the food and services will be just fine. And then inform them that all of the money that they pay for the services will be donated to a traditional pro-family lobby. If it is something like catering, where your employees have to be there to provide services, tell them that all your people will smile, be professional, and everyone of them will be wearing crucifixes and have the Holy Family embroidered on their uniforms. Then show them pictures of your uniforms. When the truck pulls up, speakers will be playing Immaculate Mary. Show them the truck and play the music.
“Oh, you would be offended by that? I’m so sorry. You approached us because we are Christians. Right? We are happy to provide services for you and we are grateful that you chose to come to our Christian catering business. We just want to be of help.”
Then tell them that you will take out an ad in the paper to let everyone know what you did with their money, thanking them by name for their business so that you could make the contribution.
The only problem is that you're still co-operating in evil if you provide a gay wedding cake. But he's on to something...