Humans are mostly thinking creatures and only mildly reliant on instinct. Even if we are "naturally" not monogamous, some of us strive for something similar because that is what we 'want.'
We have to ability to think, imagine, plan, put into action, and if all goes well... accomplish that which we set out to do.
It doesn't matter what we were meant to do instinctively when we make up our minds to be something different --- or do something different.
Human can choose to be monogamous or not... animals can't --- they just rely on instinct in such situations.
"The weakest soul, knowing its own weakness, and believing this truth that strength can only be developed by effort and practice, will, thus believing, at once begin to exert itself, and, adding effort to effort, patience to patience, and strength to strength, will never cease to develop, and will at last grow divinely strong."
- James Allen