Robert A. Heinlein is an author who wrote multiple novels with poly/open themes, most notably "Stranger in a Strange Land." Just thought I'd mention that.
Yes, most notably and famously,
Stranger in a Strange Land.
But he also addressed poly-type marriages in these other novels:
The Moon is A Harsh Mistress ("line marriages")
Time Enough for Love
Friday
What many would call "kitchen table" poly is a significant element in all of these novels.
I recently read the (authorized and reasonably definitive) two volume Heinlein biography by William Patterson (2010) - which was interesting but which would probably only appeal to scholars and die-hard Heinlein fans. So, from what I learned in those books, and what I have picked up elsewhere over the years:
All three of Heinlein's marriages were "open" - according to the authorized biography, which included extensive input from Heinlein's third wife, Ginny (or Virginia, more formally). His first marriage to Elinor Curry lasted just over a year, but they had agreed on a "companionate" marriage (a form of open marriage popular in certain circles in the early Twentieth Century). His second marriage was to Leslyn MacDonald - and they were apparently quite actively open. That marriage ended after about 15 years due to Leslyn's alcoholism. His third wife, Ginny, actually came into the picture before he and Leslyn broke up. Ginny lived with Robert and Leslyn prior to their divorce for a while. Robert and Ginny also had an "open" marriage but it was less actively open than the previous marriage with Leslyn. They were married until Heinlein's passing in 1988.
I have not been able to spend a great deal of time online during the current situation - hope everyone is well and staying safe!
Al