I mean there are so many arguments to use to defend. I hope their attorneys use them all, correctly, and well.
It would seem that a cake baker's work is really a form of artistic expression, and thus entitled to first amendment protection which should preclude the public accommodations claim against the bakers.
An artistic cake decorators work differs in nature from that of say, a simple restaurant serving food, or someone providing hotel rooms to rent.
The simple test of the public accommodations argument should be if the same Oregon commission will require every bakery to produce decorative cakes with any statement whatsoever on them - even statements that are obviously offensive. Because if baking a cake is truly just a public accommodation then any baker should be required to make whatever cake the customer wants. But they are unlikely to reach that point, in much the same way that they would be unlikely to require, for instance, a photographer to agree to photograph an event which involved activity offensive to the photographer. Or a writer to write about an offensive topic. Or an advertising agency to accept, for example, a customer whose views they found repugnant.
Perhaps dozens of volunteers should set out to ask bakeries all over the city to produce cakes with offensive messages, and carefully document how the bakeries respond. And then share the results, in the form of complaints, to the same commission.