In regard to the "open job" point that someone tried to make: we could call "cheating," "moonlighting."
I guess in poly we could just call it "relationship moonlighting" actually as "moonlighting" doesn't necessarily imply deceit.
That list of quotes is painful. I find it pathetic and painfully sad. I feel bad for them actually; that they might propetuate hate that way and bully people into their kind of relationship style. How stifflingly their inner prisons are. Gah! I wish them luck, but seriously, I feel really sad for them.