(Forgive me if this has been discussed here before):

This is an arrangement Heinein talks about, particularly in "The Moon Is A Harsh Mistress" and in "Friday." It seems to be an arrangement where you have a principle couple, who adds another person, and a little later adds another, and then another, until they pass away. Then the oldest male and oldest female become the primaries, and the addition continues along over time, amassing both new lovers but also children, and wealth. In Heinlein's universe, some of these marriages are 100s of years old, and have amassed enormous amounts of wealth over time, with the ability to send their children through college and take care of any member's needs from cradle to grave.

Seems like an ideal lifestyle to me. Any thoughts? Does anyone know of any such groups in the real world?