I've thought about this a lot, and frankly I can't see many benefits to a monogamous relationship that I can't get in a poly one. I would very much like to hear your thoughts on this subject. What benefits do you think a monogamous relationship has that a polyamorous does not?
Fundamentally, people make poor decisions because we locally optimize for apparent benefits. In relationships, the hedonic benefit of a 1:1 relationship is much less apparent than a poly/positive sexual orientation. My major premise is that the less apparent benefits exist, and should be considered. My conclusion is that for many that may choose a poly/positive sexual orientation, they are locally optimizing. I base this on what i've read in psychology, evolutionary anthropology, and biopsychology.
First the benefits of a poly/positive sexual orientation. From a psychological perspective, we enjoy things when they are novel. For example, food tastes better when you eat little bits of lots of different food. From an evolutionary anthropological perspective, men are motivated to spread seed; particularly when the available male population is low. From a biopsychological perspective, reward mechanisms are triggered during the thrill of the hunt (for a new mate), and dwelling on/idealizing the other can create a self-referential loop that potentiates the previously mentioned benefits.
I'm not making a moral argument. Morality, to me, is about doing right by others. If poly/positive is beneficial for you and those you are with, then it can't be 'immoral'. I'm defiantly not making an argument against poly-marriages. I think any household should be able to commit and share legal rights however they like, regardless of configuration. And i'm not 'afraid' of sex. Sex is awesome and I want people to have it as often as maximizes their well being.
The apparent benefits of poly/positive orientation come with costs that are not apparent in the moment. On the social psychological level we have something called "fundamental attribution"; on an evolutionary anthropological level we have something called "mutualistic interactions"; and on a biopsychological level vasopressin and and oxytocin build over time when social-sexual cues are focused on a singular individual.
I don't want to tl;dr but i'd be happy to put together 500 words (with citations) if you feel it'd be beneficial.