There is a difference between refusing to service a person and refusing to fulfill a particular request. A plumber can go to any house, gay, straight, black, white, whatever, see the request, and determine it's not worth doing. Maybe the plumber is asked to make the faucets look like floppy penises or whatever and determines the work isn't worth it. Maybe their bathroom is a complete clusterfuck and refused to fulfill that order because it's not worth it. He's not a slave to demands. He can say no to a request.
How are you even comparing to two? A plumber is a plumber. Those demands are 'unreasonable'. These demands are 'part of what they do as a business'.
There is nothing unreasonable about their design for the cake other then 'omg gheeeeeeeeeey!'. Thats descrimination.