If a restaurant, photographer, florist, baker or anyone else accepts any and all business in retail, then the have to accept gay business and black business and Muslim business without discrimination. However, if they accept jobs on a per-job contractual basis, they should be able to accept or reject any and all job commissions as they see fit.
If their religious beliefs take precedence, they should make that clear up front, because it's not something most customers would expect to encounter from a service provider. Perhaps a questionnaire before accepting assignments, just so there's no confusion about what they're willing to do.
Because, what happens if the service provider agrees to provide their service and only afterwards finds they have some personal objection - do they get to renege on their commitment? If so, can they cancel on the day of the ceremony, if that's when they realize they object? Like, say: they refuse to photograph people they consider unattractive, because it makes for terrible advertising. [At least they have a business related reason for that one!]
For example, a restaurant open to the public must serve the public. Same with a photography studio that accepts walk-ins and general appointments. But a contract wedding photographer, a caterer, baker or a florist that does commissioned contract work should be able to refuse whatever work they don't want to do.
If you can compel someone to speak, then you can likewise compel them to remain silent. But the government can't to either.
If you can compel a photographer to accept contract work, then you can also force individuals to use that particular photographer, and only that photographer, for their photographic needs. And that's the argument that should be used in front of the Supreme Court.
If I were that photographer and were speaking with that gay couple, I'd tell them, "Sure, I'll photograph your weddings. The only thing is, you think I'm a good photographer, but I'm not. I don't know how to set exposures or the focus, and am still learning how to frame shots properly. Sometimes I get lucky, but not very often. I'll photograph your wedding, but your pictures are probably gonna suck. If I remember to take the lens cap off, that is."
