Thanks... for a non-computer analogy...

For many years I thought that I was lactose-intolerant and bought Lactaid milk.

Then I suspected that I might have attributed a coincidental tummy ache or two in the past to lactose intolerance and decided to try the good old milk with lactose.

I am doing fine and do not have tummy aches.

Lactaid costs twice as much.

Lactaid comes in half-gallons only. Milk with lactose can be bought from Costco in packages of two gallons (2 1-gallon bottles).

So milk with lactose is way cheaper and I do not have to worry about running out of milk. I consume a lot of milk, so I need to have milk in the fridge all the time. So now that I can buy so much milk at once and spend so much less on an ounce of milk, I should be exceedingly happy and the change is a pure improvement.

Right?

Wrong...

There were unexpected costs and unexpected sufferers once I made the move.

I have three cats and cats ARE lactose-intolerant. My cats like milk. Back when I used Lactaid, I did not worry if they would lap from my glass (although there are horrible reports on the internet about the consequences of sharing drinks or food with cats, My cat likes to drink out of my water glass and I have been having some stomach issues. I do not allow the cat to do this Can you get sick if you drink after a cat, I personally have never suffered such consequences and when I was a teen I drank fresh cow's milk right on the farm unpasteurized and did not get botulism either, so I personally do not worry about this part).

Moreover, I experience some satisfaction from sharing high quality human foods/drinks with the cats. I like to share plain greek yogurt with them, rotisserie chicken, lamb etc. because it feels nice to have this kind of community).

Now I have to guard the glass so that they would not drink from it and I do not get that little bit of satisfaction that I used to get. Guarding the glass is a nuisance and the loss of satisfaction is a small reduction in the quality of life.

It took me many weeks of pondering to find a solution, and I am sure that the solution is obvious to you now that you are reading it, but I am a slow thinker so it has taken me so long.

I will be buying Lactaid just for the cats, infrequently, and giving them Lactaid in a separate glass just when I pour milk with lactose for myself. They will get to drink Lactaid and I will not need to guard my glass because they would be occupied.

Great.

There is a little bit of extra cost in buying expensive Lactaid, but not much since the cats won't be consuming that much.

This situation was very simple. It involved one woman, two types of milk, and three cats. Not complex, right?

Yet, a seemingly positive change resulted in unexpected costs and a solution was necessary to optimize the situation.

This is very typical of the rest of the human existence in all the areas ever touched by human decision-making. Every decision, as a rule, results in some costs and some benefits. The costs may or may not be internalized by the decision-makers etc. so it can get complicated, but the basic truth that there is no free lunch ("The free-market economist Milton Friedman also popularized the phrase[1] by using it as the title of a 1975 book,[4] and it often appears in economics textbooks.[5] Campbell McConnell writes that the idea is "at the core of economics".[6]" from Wikipedia).

So you should realize that while it is theoretically possible that monogamy is superior to all other arrangements, it is highly unlikely, because in general all arrangements have pros and cons (=costs and benefits), so if monogamy is indeed the winner overall, it will be completely unique in its status of the winner overall.

To properly calculate the overall costs and benefits of monogamy, you will need to take into account the costs and benefits to all humans, even those outside of the individual couple-units, and that is a very difficult exercise.