We also prefer to be monogamous lovers, however: The world is not black and white, one does not have to be a narcissist to be non-monogamous as their nature nor do they need be sexually addicted to think outside chruch and society dictated dynamics or your (the general "your) own beliefs and boundaries. Not everyone is of one way.
Something of interest pertaining to the subject at hand:
http://www.livescience.com/27128-polyamory-myths-debunked.html
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion