I see it more as like a deviation from the mental picture of someone you have/hanging onto in your head. Not saying its healthy but like the guy hadn’t moved on yet and is still hanging on to the memories of them together, being faithful, and dedicated to one another. Learning about multiple new partners further solidifies that their time is over and the real person isn’t the same as the person you knew them as. Coming to terms with that can be stressful. I personally think you can feel this way without actually believing you “own” the person.