I think first come first serve, and make sure you leave enough for everyone should be the rule. It seems wrong to say to singles and teens who come alone that "you are good enough to provide food, but not good enough to have an equal chance at being first - and there for the prime dishes, because you are alone." Is wrong and very rude.
My dad was a minister and made the rules for our church's potluck dinners. First, the elderly went through, then people with very young children. Then, families (and single people were a family). Teens were with their family or a friend's family, generally. If they weren't, they just went through with the family group. The chances of a 15-yr-old bringing a dish were nil, if memory serves me.
It worked well.
As a Single I would have resigned from your church then and there. And I would not be shy about answering why or taking my dish home with me ( since I don't use disposables, I would not leave a good ceramic dish behind ) Expecting one group to always have the leavings of others, is rude.
I don't get the problem. Before the "rules," kids would run to the front of the line, and take mounds of food (much of it wasted). This way parents could control what the kids took. The people who need assistance went first. I don't see the problem. The singles are a family, and go with other families. And some singles were elderly, or single parents with little kids, and went with the first groups. So they are not being overlooked.
I later attended a church where it was first come, first served. I wasn't even through the line once before kids were allowed to jump in and take 2nds-3rds. By the time I got to the table there was hardly any food left. I stopped going to those potlucks.