Historically, anthropologically, and biologically - not necessarily.
Neanderthals, which Homo Sapiens interbred with, tended not to be monogamous. Most modern humans have 2% - 4% Neanderthal DNA. At one conference an attendee was able to prove they had double digit Neanderthal DNA percentages.
https://www.nbcnews.com/id/wbna33002794
The initial attraction between humans is dopamine driven. This is known as lust aka "romance". The human body can keep this up with the same partner for a period of 2 - 5 years. After that, if a real relationship based on love does not develop, the relationship tends to end. That is why many if not most divorces happen in the U.S. if not everywhere.
It has been theorized that prior to human civilization a human male and female would get together, have a child, and when the child was old enough to walk and eat on its own the male would wander off.
Looking at early human history we find many cultures where men had multiple wives simultaneously. This brought up several issues that brought civil unrest -
- In many cultures only men could own property. A man needed a male heir to leave his estate to. Women would only be taken care of by a son who inherited his father's property. The issue with having multiple wives was "which wife's son gets the farm?" Look at the Biblical story of Abraham have sons Ishmael and Isaac.
- Men had a tendency to fight (even to the death) for the opportunity to procreate with a woman. So, when a single man had a harem of women he became a target for violence for those who had none. Once again, look at the story of Abraham. He told men that his wife was his sister so they wouldn't kill him to try and take his wife.
So, in early history, before government was used to control people religion was the tool that was used. Suddenly humans were hit divine rules - you get one wife. Don't have sex with someone not your spouse because that might produce a male heir outside of wedlock.
There is a human phenomena called generational cultural indoctrination. This is were children for generations are raised being taught that something is true, but have not been given the proof as to why. It comes to a point in time where people accept a concept as true, but have no clue as to how we got there or they refer to some ancient mythology as the continued explanation (i.e. Jehovah created marriage and performed the first marriage ceremony).
I do not think humans are naturally monogamous sexually. I do believe a lot of harm can come from not practicing it. Harm to the self and to the other.