She doesn't experience jealousy, and once a relationship ends - at least romantic - she either moves on or realises that there weren't significant romantic feelings to begin with, thus moves on easier. She even encourages Clark to commit himself to Lois by revealing his secret identity, since she sees that he loves her deeply, yet holds back out of fear, thus Diana, wanting the best for her friend, continues to gently push him. She has a lack of jealousy, but it may blind her.