Congrats on your 25th!
Sorry this has put such a damper on your cruise plans. What they told you is correct and although I truely understand your point of view, particularly with this being such a special occassion, I doubt pushing the issue with Carnival is going to end up with an exception to that rule. I don't know the situation with the "other customers" who told you they have done this in the past but I highly doubt it. This has been the policy for as long as I can remember.
Whatever the outcome, I wish you the best and hope that you are able to still enjoy the cruise and your anniversary/vow renewal on the island if not onboard.
NCL Sun, Oasis OTS, Navigator OTS, Legend, Fascination, Emerald, Glory, Mariner OTS x 2, Sovereign OTS, Celebration, Miracle x 2, Pride, Fantasy