I would have just said, “Sorry, that date is booked.”
They catch onto that, usually one person tries to book and they don’t immediately point out that it’s a same sex wedding, but once they find out there is availability for a certain date, it’s “Game Over.”
Thing is, though, people shouldn't have to lie. And if they're refusing based on being Christians, then lying is against their religion just as much as participating in a gay wedding is against it. It would also be against their religion to purposely do a bad job by taking lousy pictures. So the Christians are really stuck here, and have no choice but to stop doing weddings altogether.
We need to bring back the right for private businesses to refuse service to anyone, for any reason. Anything less than that, and we're not free.