Not beating someone up who takes your toys is a learned behavior too but it's healthier in my opinion.

Emotionally monogamy is healthier in my opinion as well. But as with any social contract, it depends on the persons involved.

One bad neighbor can ruin an entire neighborhood.

Ok, this is an often asked question from people and answers vary so much, but I'll have to say most of it is unscientific babble.
For instance
chimpanzee 'genetic' makeup is 98% identical.
By 'genetic' I'm guessing he is referring to DNA. Of which we know very little about. We may have learned to copy/paste it a bit, but we still have no idea what most of it is/does. 98% seems pretty close. Until you look at a chimpanzee. Even a slight glance will tell you there are some major differences between us. They walk, eat, look, and talk (or don't) different from us. Their entire "culture" is different from us. It's like saying a certain plant is 98% the same size as us, so that must mean that other things from it are similar as well. No. That logic is pretty flawed.
lgalatea posted some nice hypothesis with some well thought out articles too. But again, these are just opinions and most of the opinions come back around to trying to compare us to animals again. That won't work. You cannot grab one similarity from an animal and use it to make guesses about other animals. Do you see any animals posting answers on mahalo? If you really start to think of the major differences in what we do not only from instinct and from birth, but our entire culture as a whole you will realize that trying to find answers for our behavior from the behavior of other species is an effort in futility.
Another common argument involves comparing either other countries/cultures or other times in history to find an answer to a current culture in question. Some past cultures it was common to have multiple wives/husbands. Some it was not. Some it was common to kill someone who cheated on you (and still is in some current primitive cultures). There are certain countries who have imposed gun laws and their crime rate went down. There are others who did the same thing and their crime rate went up. High government activity in people's lives has helped certain countries and hurt others. What I'm trying to say is there are too many variables in the entire whole of a culture to say if one thing worked here, it will work over here as well. Each situation in unique and there is no blanket answer. So to sum up this paragraph, this type of comparison should not be entered into the equation either.

So...now that we have decided what does not work, or what we cannot draw true facts from, what do we have left. Well there are a few proven scientific things about Monogamy and people's tendency for it.
Experiments have been made on what some call "limerence", which is the chemical addiction you get upon first being intimate with someone. It could be just holding hands, a kiss, or sex. Your body begins to give you a mix of chemicals that are uppers and downers. You usually feel it most extreme the first time with a tightness in the chest as you are "injected". This high causes a lot of new 'lovers' to stay up all night talking for hours, smiling uncontrollably, etc. It lasts usually for a period of 6 months or more. However the more you experience the less time that it lasts. There is a term known as "serial monogamy" where someone who becomes addicted to this drug will start and leave relationships on a regular schedule without knowing that it is the drug they are after, not the person.
Another thing we know is that after this period of the drug wears off, if the couple stays together for around 5 to 10 years the body begins to supply it with a different type of drug. It's not a big rush happy high like the first drug but it does cause a chemical addiction and dependency toward the other person. When the other person is away, your body stops supplying you with the drug and you can experience mood swings and depression. It has been said that this could also be the cause of why in elderly marriages when one person dies, the other follows suit shortly after.
Another point I would like to make on the subject which is just my own opinion. I believe a child grows up more well rounded with a stable mother and father to raise them. This could tied in with nature making us "addicted" to our spouse so as to raise a child naturally although there is no way of knowing for sure.
Well, that is mainly what I have for real information on the subject. Everything else is pure speculation.