What do you think would happen if a gay couple asked a photographer to photograph their wedding and it turned out that the photographer was an old-style hippie who did not believe in marriage at all and viewed it as an oppressive and archaic institution, and who therefore never agreed to photograph any weddings--straight or otherwise?
Do you think the gay couple would walk away whining about being "victims of discrimination"? Do you think they would go running to the nearest lawyer to file a lawsuit against the hippie photographer? No, they would just go get another photographer.
So why can't they do that when a Christian photographer politely declines to service their wedding? Because they don't hate old-style hippies. But, they do hate Christians.
Most gays just can't stand any word or action that reminds them that their lifestyle is abnormal. They want to be treated as if their lifestyle is perfectly normal. They especially don't want to hear about God and what God says about their conduct.
There are many photographers who don't do weddings. If you don't do weddings and you say you don't do weddings, that's not discrimination. If you say you do weddings, but you only do white, heterosexual weddings, than you are discriminating. This really isn't that difficult.