As Christians, these people probably don’t want to bake a bad cake. Purposely baking a bad cake, or a cake with ingredients that might make someone sick, (X-Lax, urine, etc.,) is against their religion just as much as baking a gay wedding cake is. Besides, they shouldn’t have to bake a bad cake. They should be able to just refuse to do it at all.
So just use lots of baking powder. I guarantee no gay or other fragrant ‘sexual minority’ will desire a special queer cake from that bakery again.