One is and artistic and expressive occupation. Stitching up a gay person wouldn’t be perceived as a form of statement. But being required to produce work in the traditional style of a wedding photographer could be perceived as issuing a statement in support of the event.
If you sold signs, you shouldn’t be able to decline someone a blank sign just because they are LGBT. But you shouldn’t be required to design one that carried a pro LGBT (or any other kind) of message.