Once gay marriage is legal your analogy doesn’t stand up. Best argue on religious grounds of the business that they should not be forced to provide that product to those customers.
OK, how about...
You have a business creating custom cakes. You could, for example, make a football cake for a boy’s birthday party, or a cake that looks like a pony with a pink hair ribbon for a little girls party, etc. The sign at the door says “WE make custom cakes! Tell us what you want us to bake, and we’ll do the rest!”
Someone comes into your store, and asked you to make them a cake that looks just like a vajiji. Do you have to? How about a man’s phallus? A couple doing a sex act? I’ve seen these cakes. But do we really get to sue a bakery for NOT making them?