Agatha Christie couples also include the passionate couple who realize that they are actually also friends (Moving Finger).
Moving Finger contains one of my favorite passages from literature:
"Luke, do you like me now?"
He made a movement toward her, but she warded him off.
"I said 'like', Luke; not 'love.'"
"Oh, I see. Yes, I do. I like you, Bridget, as well as loving you."
Bridget said, "I like you, Luke."
They smiled at each other a little timidly, like children who have made friends at a party.
Bridget said, "Liking is more important than loving. It lasts. I want what is between us to last, Luke. I don't want us just to love each other and marry and get tired of each other, and then want to marry someone else."
"Oh, my dear love, I know. You want reality."
Christie was an advocate of the friendly marriage. She had a passionate marriage with the unreliable and self-centered Archie. And she had a friendly, happy marriage with Max Mallowan.
In Sad Cypress, the story of a woman who imagines killing her romantic rival, the main character Eleanor eventually chooses the reliable doctor who showed her unstinting friendship--as opposed to the unreliable, waffling man Eleanor had wanted to murder for.
Lovers-who-become-friends is a variation on this theme. They have the passion but do they have anything else?
My favorite M/M couple makes another appearance here since Silas and Dom's relationship in KJ Charles's Seditious Affair begins as purely sexual--a safe place for Dominic to get the domination he craves during sex. At some point over approximately a year, the relationship transforms as the men begin to discuss books, a topic about which they are nearly as passionate.
What about the other way around--the friends-who-want-to-be-lovers. Is it an automatic, "Yes, of course it will work!"?
In the below quote, Dom explains to his good friend, once his lover, why their relationship would not have lasted--despite their great friendship.