I could see thinking about this differently.

As people quit believing in God, they stop thinking in terms of "God brought us together/we were made for each other" (though they stop thinking that a generation or two later than they stop believing in God).

If you think that we made this relationship, then maybe a lab-grown (human made) diamond fits? (Though it may take an advertising campaign before people see it that way.)

Disclaimer: I'm not a sociologist. This is just my speculation about how the dynamic could change.