There are the ones with the strings, knowing who they will be with.
There are the ones without the strings, knowing they will be alone.
There are the fates, ones who can cut the strings.

Everyone sees the strings. Everyone know who they will be with, or if they will be alone. Couples are formed young. To date anyone else if you have a string is taboo. Stringless will date each other, staying friends, knowing it won't last. Friendships form, and everyone finds a place. Everyone knows the worst tragedy is having your string cut.

Strings form when the second one is born. Any age, or any gender. People understand.

One boy was born with a string. Across the hospital a weak three month old was sick, and was getting his shots. His small body not able to lose much blood. A small, weak string forming on his finger, just for a moment. That was the day the new born cut his own string.