Originally Posted by Emm
As do (off the top of my head) Friday, Time Enough for Love, and To Sail Beyond the Sunset. Much of his later work could be classified as either "strong poly theme" or "soft porn", depending on your mood at the time.
Like that description!
Recently reread "Stranger..." and also "I Will Fear No Evil" - both seem rather poly... Heinlein's fiction did rather favour idea of older men and eager young women though....
Best poly attitude I've noted in SF was Vonda McIntire's "Dreamsnake" which imo is a fabulous read and has a marvellously inclusive/feminist perspective on life.