Sure. I'd say that would fall under a reasonable man standard and, unlike accommodations arguments, it's requiring you to go and work in what might be perceived as a show of support, endangering your business.
Now how about this one: should a hamburger joint be allowed to post "No N-word Served here!"?
Anyone going to give that a shout out?

lain:
I doubt it. So there's the reasonable and the unreasonable and they aren't really about a class of people, but a course of conduct and its impact on a business model.