Monogamy is a commitment that may not be the most logical choice for everyone.
Some other sources have suggested that monogamy runs counter to the purpose of genetic diversity.
Attraction and longing are rather superficial elements when stacked up against the emotional intimacy and commitment of a relationship shared, with or without the rearing of children.
Hormones and biology are powerful influences during puberty and young adulthood, and perhaps some make commitments that they are not mature enough to comprehend the significance of, merely to satisfy the urges of biology.
Then there are those who elect not to commit for personal reasons of not desiring to be bound to a commitment made.
We live in a time when persons have far more options in regard to the type and duration of relationships they wish to make.
I was raised with 'old school' teachings in regard to commitment.
If you choose to commit to a relationship, then that means a total commitment.
It does not preclude that some day you might both find yourself grown into different people and agreeing to go your separate ways, respectfully, to search new paths of fulfillment.
It is even acceptable to glance at the menu, provided one remembers that they are dining at home, lol....and no sampling the 'salad bars'....
The topic under discussion was given an interesting treatment in D.H. Lawrence's work, Lady Chatterley's Lover. Some interesting social circumstances were addressed at the same time.
http://en.wikipedia.org/wiki/Lady_Chatterley's_Lover