If the bank has to let nazi bronies write a check to pay for a nontraditional wedding cake, does a baker have to accept the job?

How about this: Persons doing business as persons can use their judgement and contract to do jobs as they please. People representing corporations, and the corporations themselves, can't.

The Public licenses corporations to exist and operate, one of the terms can be "people made of meat shall be served by your corporation without regard to the flavor of that meat."