I can't believe no one mentioned Casablanca or Gone with the Wind.
But, yeah, when the conflict of the plot is a love triangle (or quadrangle in GwtW), resolving it with a happy poly situation destroys the story.
I'm a fiction writer. Story = conflict.
If Anna Karenina's husband could just accept a co-husband, there wouldn't be much of a story...
Single, straight, female, solo, non-monogamous.