All I Do Is Dream Of Two (смешанная)

"And then Don and Kathy married and lived happily ever after with Cosmo" is how I have summed up the end of "Singin' in the Rain" since I first watched the film as a child. But how did the triad came to pass? Who made the first move?
Here's a scene that takes place a short time after the film.
Happy Yuletide!