I used to be a cake decorator, so I understand your sentiment here. Though being in a corporate setting, things like copyright and derogatory terms and words weren't allowed. Thought I would always try to work with the customer, so we could agree on a design that wouldn't get my company sued.
Though I don't agree that you would make a cake that celebrates something so hateful. Price means nothing if you're doing it to support hate.
Yes, but a cake in the shape of a bible that says "God hates fags" is hateful, not just discriminatory- it crosses the line. As a business owner, you would think to know that supporting and providing service to such an awful, judgmental point of view is not only hurtful to society, but also possibly your business venture as well.
It's interesting that people think that being professional means that they have to ignore their morals. Especially if you've making money off of people celebrating their hatred of homosexuality.
Again, I bring up morals. You can tell a customer "NO" any time you feel is necessary. Let them complain, they can't make you do anything you don't personally believe in, or believe you're allowed/obligated to do. Using professionalism as an excuse for making money off of hated is unacceptable.