"What is a marriage to you?"
"Marriage is a socially or ritually recognized union or legal contract between spouses that establishes rights and obligations between them, between them and their children, and between then and their in-laws."
(quote from Cultural Anthropology: The Human Challenge -13th ed. 2011) - Wikipedia Definition
"socially recognized": the community formed by users with a Borderless Citizen ID could be the social actor that would recognize a Borderless Marriage.
"legal contract": smart contracts on the Expanse blockchain represents the legal contract.
"between spouses": it does not reduce marriage to a contract between two spouses (neither specify about the sex of each spouses, that refers to the definition of "spouse", so Borderless Marriage can allow marriage between several persons of different or same sex.
" that establishes rights and obligations between them": these rights and obligations will be operated through the smart contract. For instances, the obligation for sharing expenses due to the smart contract, proportionally or not to their wallets, and the right to maybe gain some sort of reputation bonus?
"between them and their children": if a user has a Borderless Citizen ID and his parents also have one and are linked by a Borderless Marriage, then the user could be integrated in some way in this marriage contract and maybe gain some reputation advantages and basic needs guaranteed in some ways?
"and between then and their in-laws.": of course we can extend the guaranties of being included in a Borderless Marriage to in-laws and extra users in special situations.
It looks to me like a laic, secular view of marriage, a definition based on the social aspects of being married, not on the ancestral aspect of one male and one female linked by a pretended natural or even deific law. And that also is the way I would appreciate marriages to be on a digital way, "open and inclusive" :)