Dating a friend from childhood
The Childhood Marriage Promise is often made in the shadow of The World Tree. When it's the parents who make such an agreement, you have the decidedly-less-heartwarming Arranged Marriage.
When two adults make such a deferred arrangement in order to avoid being single in old age, it's a Fallback Marriage Pact.
Occasionally in later years, these friendships are "rekindled" following separations or passing of their adult partners which lead to a later life marriage, union etc.
These instances are notable as they are made popular by media coverage.
A slight variation, one that usually comes up in Fantasy or Sci-Fi anime, is where the boy promises to always protect the girl (which, in the traditional sense of marriage, is pretty much the same thing). See also Declaration of Protection, which can be read both ways.
Often arises in a Forgotten First Meeting scenario.
At the time, neither one really understood the implications of that vow.
Between the Titans of Myth and Dark Angel, the two parted. Husband twice as old as wife, Augurs ill for married life; Baleful prophecies were rife, Twenty years ago!
In ideal circumstances, the term applies mutually to both parties and corresponds both ways, hence the plural being childhood sweethearts.
The relationship may involve romantic love or may be an extension of a close friendship.
Often, intimacy by way of kissing will occur in order to show affection.
The quality of sharing is not the same as in adult friendships.
However, in a certain way the friendship has a deeper quality.