Photographer refuses to take photographs at a gay wedding because of religious beliefs. Will take photos of any other ceremony.
But the photographer probably wouldn't take photos of every other type of ceremony, either. The photographer would probably refuse to photograph a satanic ceremony, for example.
My wife is a photographer and has shot several gay weddings. We obviously do not find gay marriage or providing services for gay weddings to be objectionable.
But, I think there's a difference between providing a service and selling a product.
If you're baking a cake, it's just a cake. You're not baking a gay wedding cake or a straight wedding cake. It's just a cake. And if you refuse to sell it to someone because they're gay, well that's discriminatory based on the sexual orientation of the client.
But if you sell the service of creating wedding photographs, you are specifically creating photographs that feature a gay wedding or a straight wedding. If you object to gay relationships on religious grounds, you're now being forced to create images the existence of which you find to be immoral.
There's a difference between refusing to sell a product to someone because you believe their actions are immoral, and refusing to perform actions yourself that you believe are immoral.
I think a photographer should be allowed to refuse to shoot the gay wedding, and that people should then loudly denounce them for it. But I don't believe they should be forced to do something they think is immoral.