Like everyone before, you did not plan this to happen and it's unfortunate but rules are rules. Whether you have Carnival's insurance or someone else's it's a necessary evil that's worth every penny. Losing a favorite couple such as this definitely puts a damper on things. I'm sure your BM did not plan his clot and feels terrible. Remember his healthy when you're venting, he may feel worse than he already does. His wife too. What would you do if you were the other couple? Think how they feel.
The alternative, without filling the cabin, would have been to enjoy the cruise by yourselves like a second honeymoon. Ask to be seated with other honeymooners and make the most of an unfortunate event. When you returned home, you and the other couple could have celebrated with our extended family and your BM would have been so pleased to have been able to join you for the renewing of your vows and the celebration.
It is what it is. Hopefully you do have insurance. Cancel the trip, wait a couple of years and then laugh about this with your bm and his wife.
Not that this isn't a tragedy so to speak, but think about the other cruisers the past couple of weeks that may be coming home to flooded homes or no home or that have had their itineraries altered. Think about how all of us look soooo forward to our cruises and people such as myself have been on cruises where they've missed a port due to mother nature or a mechanical dilemma and you make a good time out of a bad situation.
Don't give up on cruising, just take a step back. Someone else's situation is always worse than yours. What if someone tragic happened to your BM if he had decided to come on the cruise... you would never forgive yourself.