Unfortunately, based on what you've said, I highly doubt that any appeal in this case would be successful. They simply see that you are not eligible to be sponsored because you have not completed your marriage rites and therefore are not fully married to the CIC. I'd advise waiting until after you've observed all your specific ceremonies, document carefully, and then reapply. To be completely frank, you're extremely lucky that this process only took 3 months. Many people wait months, or even years, to get an interview. Establish what you need to ensure the CIC sees you as a bona fide married couple and then reapply. As it seems that the rest of your relationship proofs were good for the CIC, then I would imagine that your next application will be much more successful.