Finally got around to seeing Best Gift Ever. I had to stop and rewind to make sure I saw what I did. At the very beginning there is a shot of Big Mac and Sugar Bell and in the back ground is Marble looking like she is upset.
One episode it was only hinted at that Marble and Big Mac might like each other.
Why put this in the show?
There was a bit of fan shipping in the community that the show runners may have been aware of, and they may have just wanted to show it wasn't happening. Making it so depressing was perhaps unnecessary, but it's a part of life so worth showing it happens and you can get over it like Marble did it the next scene.
Yeah, I'd say it's just to stop the shippers.