https://en.wikipedia.org/wiki/Time_Enough_for_Love
which is popular among poly people but is one of the very Heinlein books I didn't read when I was a kid, maybe because being the story of a 2000-year old man it's crazy long. (It doesn't help that Heinlein had serious health problems after he wrote "The Moon is a Harsh Mistress" and that everything he wrote after that had quality problems)