It came out of a running joke in the fandom to Marco being the most shipped character on the show, and how often he managed to have romantic implications and chemistry with multiple characters. This huge poly ship is not really taken very seriously within the fandom and most fanart or work for it is used in a comedic sense. But it does prove one thing: Everybody loves Marco.
Characters Mainly Involved Edit
Star Butterfly Edit
Star was the first to enter Marco's Harem, being the main character and being the most popular and best character to ship with Marco.
Jackie Lynn-Thomas Edit
Jackie didn't enter harem status until she became Marco's girlfriend in "Bon Bon the Birthday Clown". When she became shippable with the Diaz was around the time the harem started.
Janna Ordonia Edit
It's always been suspected Janna had a thing for Marco with all the tricks she pulls on him.
Tom Lucitor Edit
Tom became a candidate for Marco's Harem ever since "Friendenemies" became a thing. Having Marco and Tom star together in their own episode would've inevitably sparked a ship, but an infamous promotional poster really kicked off the ship.
Hekapoo entered Marco's growing harem after "Running with Scissors", where she sent Marco through an ultimatum to get his own dimensional scissors. Many fans suspect they would've had children with each other at the end of the episode had Star not intervened.
Kelly was barely considered a character before "Lava Lake Beach", but now that she and Marco had an intimate moment in that episode (sharing a hoodie), she's been heavily shipped with the Diaz.