If you find some unreasonable bigot who thinks you are an Affront To God, the reasonable thing to do is to take your dollars elsewhere, (whether or not they're actually willing to take you on as a customer). Of course, sometimes it might be impossible or impracticable, in which case it is fair to ask for state to intervene.
If I was a judge in the case, I'd find out what it would have cost to the couple to order a similar cake from elsewhere (including additional cost of time and transportation) and have the baker cover the difference. However, unless that difference was significant, I'd leave the couple to cover at least a portion of whatever legal costs they incurred.