For a while I was actively editing the Polyamory article on Wikipedia. I split off the list of famous polyamorous people into a new article and Hienlein was included in the list before I got there. Another editor subsequently came and decimated the list citing Wikipedia's rules about requiring sources. Heinlein and a whole bunch of other people were removed at this point. None of the editors of the articles have been able to find any sources to say the Heinlein was polyamorous in actuality so he has never been added back to the list.
He is clearly a foundation block upon which the modern polyamory movement are built and is cited by many of the multi-decade polyamorists.