I'm a photographer, and I know how to take pictures of beautiful women (brides), and romantic scenes and all that. But, a gay wedding has to be very different... with the normal roles being not defined in the same way, and with the lack of a normal couple.
If I were to shoot weddings, I would specialize in normal hetero weddings. If you want to marry a tree, or marry someone of your own gender, than I'm not your best pick, and I'll be honest enough to decline the job. Heck, I'll even be happy to hit the web to get you a dozen or so other choices better suited to your special requirement.
I really don’t think the law can or should force someone who does artistic or entertainment-based work to do something they don’t want to do. Is there any precedent for that? Can I as a man sue to get a job at Hooters, to compete in Miss America, or to be cast as Juliet in Romeo and Juliet? Are they not allowed to say that my gender disqualifies me from being a candidate for playing a role in the entertainment or artistic product they’re making?