What are the motifs in The Killing Game by Iris Johansen?

Being in love with more than one person is a recurring idea. Eve Duncan cares for Joe Quinn and John Logan. Until The Killing Game, Joe has been married and not free to have a sexual relationship with Eve. Joe and Eve are honorable people who won't become involved while Joe is married. Eve has been with John for a year but isn't ready to marry him. When Joe comes into her life with a new skull to identify, Eve is ready to go with Joe. After she finds out that he is no longer married, Eve turns John away. She won't allow him to be with her any longer. It hurts her to hurt him. Can she be in love with both men? Is it possible to truly love two men at the same time? Is it just the American culture that claims that we should be in love with one partner at a time?