I saw this one when it came out and am summarizing from memory, so hopefully my memory is accurate.
There's a made for TV movie from the nineties starring Farrah Fawcett, Lea Thompson and Peter Weller called The Substitute Wife
. I think it deals interestingly with a number of poly issues.
The wife is dying and wants to find a replacement wife for her husband. She finds a prostitute played by Farrah Fawcett who wants to get out of the business. Farrah's character agrees to marry the husband after the wife dies and comes to live with them, on their property in separate lodgings, and the husband courts her and falls in love with her. The wife, who is unable to perform sexually, encourages her husband to become sexually involved with his prospective bride, they do, and then the wife's illness goes into remission. The husband and wife resume intimate relations. They deal with the fact that he and Farrah's character still have feelings for each other and live in a poly V for a time until the wife's illness unexpectedly returns.
I think that's pretty poly.