I think the most important point to consider is that there comes a point in life where an individual and/or couple should begin to create their own reality- independent of culture and society. An objective view of the "reality" of monogamy easily shows that it it is simply not that successful. The Fairy tales, love songs and idealistic ideas of long term monogamy are just stories and lies for the most part. That can be difficult to admit at first, but once an individual or couple admits it and goes through the grief that comes with letting go of a dream or an ideal- there is a freedom and with that freedom comes the opportunity to make personal decisions which are no longer based on the fantasies of society and culture.
The key to life is in being fully engaged and peacefully detached simultaneously and authentically in each moment.