Deep Spirits

Before I get into this one I'm gonna define a few terms, a couple aren't quite being used accurately but I'll explain why shortly:

Polygamy
A form of marriage where a person has more than one spouse at the same time.

Monogamy
Where an individual only has one spouse at any one time.

Polyamory
Wiki gives me "Is the practice, desire, or acceptance of having more than one intimate relationship at a time with the knowledge and consent of everyone involved." though this implies that the relationships are separate from one another and that's not really what I have in mind for this discussion so here we'll think of polyamory to mean an intimate romantic relationship between more than two individuals.

Monoamory
I actually couldn't find this one in the OED or anywhere else which struck me as a bit strange given the previous term but oh well. Here though we'll use this to mean a romantic relationship exclusively between just two individuals.

Polysexuality
This term and the next actually refer to a sexual orientation but if it's alright with y'all we'll ignore that here. The point of this term is to draw a distinction between romantic relationships and sexual relationships, obviously the two can be combined but the important point is the two can be entirely independent of one another so we'll use this word to refer to sexual relationships between more than two people.

Monosexuality
And of course this refers to a sexual relationship between only two people, there's probably little point to these last two on their own but the idea is you can use them to highlight a distinct relationship paradigm wherein two partners are romantically involved and romantically inclusive but still accept and desire to sleep with other people.

Right, just thought I'd get those quickly out of the way as they usually come up during this subject.
So on to the topic at hand, the above are three totally interoperable pairs of alternative relationship paradigms, and I'm sure there's many hundreds more factors that can be thought of too.
Of course we can define the mainstream paradigm which in Western society is almost universally monogamous, monoamorous and monosexual. Which works to many degrees. I'm just wondering how the folks here at DS have strayed from the typical paradigm during their lives and what we all make of the alternatives.

Do you believe perhaps that the mainstream idea is the only idea that works? Are your thoughts on the matter perhaps held by your metaphysical views? Why do you think the mainstream idea is what it is? Why is your own paradigm the way that it is? Why do you believe those that live by the alternative options do what they do?

Interesting topic and the idea of title you gave to it.
All these terms you have presented are words that have appeared as a result of human need to describe all kind of human behaviour that has shown the variety of choices.

„I am a human being, so nothing human is strange to me. “Latin comedy writer Terentius who lived from about 190 to about 160 B.C.
He is often named "Terence" in English.

I know people who belong to each of terms you gave and I have never had problem with any of them. Why should I? I do not consider myself better or anything else because I belong to the mainstream idea, if under this idea you consider those who are monogamist.
I know people who are very happy and they are not monogamist.
A woman who I used to know very well surprised many around her when she married an Arab prince becoming his third wife and becoming two kids with him. I was not surprised. Knowing her I knew that she has just followed her heart however strange that could have appeared in her family or between friends. She is still happily married and from her I was able to hear many interesting things that put completely different light on this kind of relationship than we are taught to have.
I also know people whose choice is Polyamory or Polysexuality. Again, I think that those are personal choices made out of personal reasons that we generally and superficially tend to judge being caught in the mainstream idea. And those people I know are also perfectly happy with their choices.
So, my answer is that the mainstream idea is not the only one that works.
Asking why those who live their alternative choices do that is for me the same like asking me why I live my choice. I live my choice because I have liked it like that.

I don't think it matters in the slightest except when a child is subject to the happenings. As long as a person is making decisions for its own life without subjecting another to it in contrary to its own best interests... it simply doesn't matter... In my opinion...

As for me... I have a child... I helped bring this child into this world without its consent so I am responsible to make sure I do everything I can to ensure the child has the tools it needs to "live long and prosper".