Marnie knows that Marlon would be a great match for her but she's too involved with Lewis to commit. However Marnie might not know that Marlon already has a work husband in Gil and she may not like finding Marlon already has that full-time commitment. And remember that Gil is not a person, instead it's an honored title bestowed on the current:
Suppressor of Rocking Chair Movement
Wearer of The Hat of Stillness
Conserver of Energy
Delayer of the Universe's Heat-Death
Bestower of Tasks too Untoward for the Monarch of Restiveness
King, Queen, Knight, Jack, and Bishop of Laziness (he unified the belts)
Winner of Staring Contests with Stone Cats
Discoverer of the zero state of Brownian Motion: Blackian Serenity
Marlon envisions himself as the next of Gil and I don't think Marnie has what it takes to deal with that.
