one taco costs 33 gil

The Twelve also has Nymeia as the spinner and the goddess of fate, so I think it would make sense that the Twelve would have ceremonies and believes centered around various senses of eternity. Sort of various cries for kings and emperors to live for a hundred, thousand, or ten thousand years; statistically unlikely, but still something they shout out.

The only time marriage traditions like that comes up, I think, is in Coerthas Central Highlands in the side-quest "When a Man Loves a Woman":

Lancefer: When a widowed man or woman takes back the ring of troth, the dead are free to wed again in the afterlife. She was young still and merry of heart. I would that she finds a mate again, though my heart cries out for her to wait for me…

So, at least in the Ishgardian branch of the Twelve, they seem to believe that the ring represents them being together in the mortal life and the afterlife, yet they also believe that there is a possibility to take the promised eternity back, to "free" the dead, as it were, to find a different eternal love.

There could be a mixture of "the ring is a symbol of our bond, that will be potentially eternal as long as we stay faithful and carry the rings" and "this ring symbolises that our bond is the Spinner's will, and we will find each other in every life."

So for me, it seems like the general belief in the Twelve's religion is that the Ring of Eternity is like an invisible handholding (which is funny since in-game the ring helps you teleport to each other free of charge); as long as they carry the rings, they are bound together by fate, and there are rituals to let go of the hand, so to say, though it seems like the power to do so lies within the living, even though the dead seemingly can then choose to wait for the living to join them once more.

The thing about marriage as a historical institution is that it's heavily tied to property rights, so as romantic as it might sound to be "eternally bonded," it doesn't really work in practical terms because remarriage is not only something that is frequently desired, but an economic and political necessity. To hold that marriage carries into the afterlife carries some difficult personal and theological implications for monogamous societies. (In this video ex-Mormon Alyssa Grenfell talks about how the LDS doctrine of eternal marriage makes polygamy a theological necessity even if it is no longer practiced in this life, and also the gendered double standards the church has written into that theology.)

had to stare at this for a minute before it clicked

Esther Dust for @esteck-art