Seems to me that in these type of edge cases (which may require a court to decide), the rule should be based upon if the person requesting the service is being discriminated against. If you WOULD make that peephole bra for the town's crazy grandma, but NOT for the dodgy-looking dude who you think might be a pedo, then you're discriminating. If you wouldn't make it for anyone, then no-one's rights are being violated. Weird example but I hope you see what I mean. [edit - to be fair, I'm not sure I agree with myself on this. It's a tough issue]
I do see what you mean. You might argue that you only make that bra in an adult size. Likewise you might argue that you only make wedding cakes with flowers/non human figures. And if you did so, fair enough that you wouldn't want to put two brides on. There are some religions that prohibit the use of human images anyway (strict islam being one). Also to throw into the mix: you're a wedding caterer, you're Jewish, you don't make or serve food with pork. Someone asks for sandwiches with ham instead of salmon. Are you required to make those? If the wedding cake guy said: "I can't make you a cake saying "Ben and Steve's Big Gay Wedding", but I can make you cake with flowers, or one with a generic "Happy Wedding"" - would that be okay? Then back to the first example, you could offer to make the dodgy-looking dude a bra, but only in an adult size or above. Then you have the dilemma that his wife is a midget/little person...