As civilized as we think we are, our biology still compels us to do primal acts. Some people are better at repressing their primal desires than others so that they may sustain a monogamous relationship. Others succumb to temptations that come from hormones and the human biology. It appears that statistics show us that although we may intend to endeavor to be monogamous, things change. This leads me to believe that the concept that is more difficult to sustain is the “happily ever after” model. Monogamy is fulfilling, rewarding and safe until those attributes no longer exist. Throw in the promise of some exciting variety that is absent from the current relationship and biology is there to feed the fire.
I believe the answer to your question is yes and yes. Humans are monogamous by nature, but since we often do crave multiple sexual partners, we are only monogamous for a period of time that sexuality is fulfilling, rewarding and safe. No doubt many other factors influence monogamy, but few rise to the power of sex.