As a Christian, I don’t care. There is no command that says not to lie. There is a command that says not to bear false witness against your neighbor.
If necessary, I would just tell a gay couple sorry I can’t do any weddings that day and make up an excuse for back-up days.
Maybe a good way to handle would be to explain why you are against gay marriage and then make the cake. Then through the entire ordeal, just continue to tell them how much you disapprove and do it ate very opportunity. They would rue the day they asked for the cake for sure. LOL!