While i think it is morally reprehensible to deny business to someone because of their sexuality, forcing someone to work for any reason is essentially just as bad. It's no different than forcing an employee to work overtime when no legal contract is in place.

If it were a doctor it would be different, but it's a non-essential baked item.

They don't want to work. Do not make them work. Let their business die and find another bakery.