By analogy with an Idiot Plot which dissolves in the presence of smart characters, a "Muggle Plot" is any plot which dissolves in the presence of transhumanism and polyamory.

Shortly after generalizing this abstraction, someone at a party told me the original tale of the Tin Woodsman, in which there are two men vying for the attention of a healer woman who gives them replacement metal body parts while constructing a whole new body out of the spares. In the end, she decides that the men she's been healing are mechanical and therefore unloveable, and goes off with the new man she's constructed.

"Ah," I said, "a Muggle Plot."

They're surprisingly common once you start looking. I originally generalized it while watching the romantic subplot in Madoka. Blah blah, not a real human, blah blah, love rival..