Who are the childless adults, and why are they childless?

In the dystopian society depicted in The Giver, adults exist to take care of children given to them by the state. They do not conceive these children and do not form any kind of emotional bond with them. Once the children have grown old enough to go out into the world on their own, their parents are pretty much redundant; their work is done.
As the parents were chosen by the state rather than choosing each other, they go their separate ways once the children have flown the nest. They go off to live with other adults in the same situation, so-called Childless Adults. In this way, the state ensures that there's no such thing as a broken home. Unfortunately, it also means that there's no such thing as love between parents and the children they care for.