I second both posters, especially Nyc. What terrible emotional calamity would befall this person if their partners had other partners? Nervous collapse and an inability to get out of bed for a week? Uncontrollable vomiting and non-stop weeping? Or would it just be painful? What makes the person so sure the pain wouldn't fade? And what makes them incapable of coping with a little pain? Obviously they've got some decent coping mechanisms to be able to handle their own hypocrisy. Why not try to grow instead of limiting others they allegedly feel for?
This isn't an issue of "true" versus "fake" poly. This is an issue of selfishness versus integrity.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.