No one invented our quasi-monogamy. The behavior of all animals with moderately complex brains are driven by emotions. Behaviors that on average result in more genes being passed forward in time persist and therefore the emotions that drive these behaviors persist. Behaviors that on average pass fewer genes into the future slowly decline and therefore, after hundreds of thousands or even millions of years, we end up with all of the species inheriting the repertoires that on average conferred a better chance of passing on the all-important genetic payload.
Today, surrounded by amazing technologies we neither understand nor can fully adapt to, we're basically cave-people surrounded by shiny buttons we press with all the fervor of laboratory animals seeking the experimental reward. Monogamy is merely one of many attempts our species has made to reduce the level of violence that results from males trying to engineer mating opportunities. It is hardly a global phenomenon (orthodox Muslim societies end up with a few men having multiple wives and therefore de facto enforced celibacy for a great many socially unsuccessful males, which in turn results in higher levels of violence than we see in societies that attempt to make monogamy the norm).
Moreover, the behavior of women and men is the result of the economics of reproduction. Eggs are few, gestation is metabolically expensive, and child-rearing is a long-term resource-intensive commitment. So women must be choosy. Men produce millions of sperm and the energy expended in mating is trivial, so men on average will seek intercourse with any female of childbearing age. So much, so obvious. But as social status also governs primates like us, it's clear that those near the top of the local hierarchy will have the opportunity to be more selective. So competition for mates will require cunning and guile as well as great genetics and a healthy body. Women must seek a provider (probably a lower-status male trading resources for mating opportunities) yet when ready to conceive attempt a secret tryst with a genetically-gifted male so as to maximize her chances of her 50% of the resulting genetic payload being passed forward in time. In consequence of this, once again behaviors are hardwired and driven by emotions. Indeed, so powerful are these behaviors that they can lead to a person believing one thing and doing the opposite, while being entirely unaware of the disconnect. Because we persist in believing fairytales about "free will" we tend to take other people's behavior personally instead of seeing it for what it really is: a largely involuntary set of actions resulting from neural hardwiring evolved over enormous periods of time. This misunderstanding creates huge personal disappointments and malformed social policies. We'd be better off if we realized just how little control we have over our actions and therefore craft social policies - and expectations - more in line with what we really do, as opposed to the Hollywood-style fantasies we carry around in our heads.
