My husband and I, have been together for 10 years. We started out with the intention of being non-monogamous. I had been married before, and had swore to myself, I`d not be 'owned' again. My ex really disliked that I was bisexual.
Of course, I had no idea how deeply I would love my husband, or what a trip it would be , when I fell in a puppy-love with my girlfriend I had 9 yrs ago. I still didnt relate any of that to a word called ' polyamory'.
We referred to ourselves as 'closed, exclusive swingers'...we would toy shyly with our feelings, and make remarks, but there was no big, proclamations.
I learned so much from that, though. We didn`t realize we were 'odd'. That we hung out with our 'friends' for all types of vanilla get togethers, had sleep overs, our kids all played and we celebrated birthdays, and holidays. It just seemed natural.
We have had periods where we chose to be monogamous, and periods where we choose to be in a open relationship.
I think nearly anyone we have dated, (minus current quad-like tendencies)have been people who started out in monogamous relationships/marriages, that opened up between years 8-12.
Thats just my findings though.
It wasn`t until we started looking for 'polyamory' versus 'open marriage' as a search, that we found the opposite to be a majority,...childless couples, or people starting out non-monogamous.
Geography plays a big, big part.
I`ve lived in two parts of my country, and the differences are vast. There is also a difference in mentality, with small town, versus big city.
Again,..just my experience.
The longer you are into it, the more ebb and flow of various types, you will encounter.