The answer may surprise you.
It’s not the ring, the dress or the ceremony. It’s not the preacher, the church or the gathered friends and family. It’s not government sanction or the filling out of the correct legal forms. It’s not the taking of vows or the proclamation of banns.
We do all that stuff, and there are sound reasons not to discard most of these customs. One is foolish to spurn the accrued wisdom of generations simply for the sake of novelty. And there is value in the blessing and support of family and friends. There is strength in community. As Immanuel Can pointed out recently, marriage is hard and we need all the incentives we can gather, especially in this individualistic age, to remind us to take it seriously.
But not one of these trappings is essential.