So if we opt to be monogamous, why do we ultimately cheat? The answer is quite simple. People do not cheat because they're pigs, sows, bitches, or dogs. It all comes down to two basic drives: the physical sexual drive and the emotional need.
People usually cheat because there is a conflict between their physical and emotional desires. By accepting and understanding these shortcomings -- instead of ignoring them -- we can hopefully work harder to make sure that our partners are satisfied enough to resist any instinctual sexual urge.
The question you have to ask yourself is which drive is stronger, and which one has a bigger influence in your life. In general, each person is different, but it is generally the physical sexual drive that dominates a person's actions.
Why? Because this drive has been present in human behavior for millions of years. Whereas the emotional monogamous need has only been around for a few thousand years, obviously a few thousand years of emotional needs will not overcome millions of years of one's evolutionary sexual drive.
Throughout history, men have argued that it is in their biological nature to desire multiple partners. Over time, we have evolved towards taking a partner in order to help raise our children and enforce a set of moral codes that contradict our stronger physical needs.
Humans are not monogamous by nature and when we ultimately choose to be faithful without the right conditions in place , we are setting ourselves up for failure and disappointment.
Once we can accept that 10,000 years of social monogamous behavior cannot supersede millions of years of physical evolution, only then can we learn to work around our weaknesses .
We exist as human beings on two levels: with bodies (physical instinct) -- the stronger of the two (according to Maslow's Hierarchy of Needs) -- and minds (emotional needs). In order to understand how physical instincts and emotional needs interrelate, we need to make a few comparisons.
Imagine that the sexual physical instinct of a person (both men and women) is the sheer brute force of natural flowing water (one of the hardest elements to contain), and the only thing that can control it is a super dam. The dam represents the human's decision to become monogamous. It will only work if the dam's foundation is built strong enough. If there are any cracks or weaknesses, the "water" will eventual break through the "dam."
In short, humans have set very difficult (but attainable) objectives for themselves by choosing to be monogamous. Therefore, in order to contain our sexual drives, we need to ensure that the emotional support toward our lifetime partner is strong enough to keep our primitive instinctual urges from surfacing.