The concept of standing has been widely criticized. Richard Epstein has a good article from over a decade ago, about the problem with the current doctrine on standing.
From that article (found here, emphasis mine):
At this point what establishes the standing as a general matter? One obvious answer is that the standing requirement is met when it is shown that the plaintiff has suffered “harm,” even if the other elements of the cause of action are not satisfied. For the moment, I shall put aside all the heavy freight that is attached to this term, and content myself with the observation that the Supreme Court has understood the doctrine of standing, and in good tort-like fashion tied the doctrine to the frequent cases where individuals seek redress for personal grievances, usually by invoking principles of corrective justice — that is, those principles, compendiously described that stress the wrong that this defendant has done to that plaintiff. But its effort to locate the doctrine of standing with tort doctrines has led to the fatal mistake in this area, which is to ignore a second class of cases, which commonly were brought in the courts of equity in which a single individual sues not for individual redress, but to preserve the structural limitations found in our or any other constitutional order.
But even so, he his point here:
And last, again in terms that echo the common law requirement for proof of damages, the harm “must be ‘likely,’ as opposed to merely ‘speculative,’ that the injury will be ‘redressed by a favorable decision’.”
One need not be actually damaged, but that damage be likely. I don’t think it unreasonable in this case to think it likely that a graphic artist–or any other artist or vendor–will be charged with refusing to provide services at a gay “wedding”.