Unfortunately, as is always the case when dealing with diverse experiences, there is probably a range of definitions to be contended with.
In my case and the case of most but not all of my single friends, single is a broader definition to mean a person who is not in a long-term committed partnership. A single person may be dating, or could be seeing someone, etc. In a poly context, that definition is often expanded to a person who has no primary relationship, though many secondary relationships can be both long term and committed.
The trouble is that this is something somebody self-identifies as. It's not a list of conditions that must be met in order to qualify. The marginalization you speak of comes not from taking a definition other than the one you describe, but by taking that very definition you are describing and dismissing anything that falls outside of that definition.
So the idea of several single, poly-oriented people coming together and a practicing poly relationship forming by mutual agreement from that, seems like it would be a very rare thing. But the idea that an initial two individuals coming together with the agreement from the beginning to leave the relationship open to poly seems feasible. It is (in my mind) like comparing searching for a unicorn to developing a V within couples.
I'm not sure I get that last sentence about comparing unicorn hunting to developing a V. Could you elaborate on that?
Other than that, I think several people coming together to form a poly group is only one of many models a person can start a relationship with. I've known at least two triads and one quad to begin this way, and I have no idea how they managed it, but yay for them! For myself, I am interested in building loving open, one to one partnerships.
Unfortunately, most poly communities are full of couples who are unicorn hunting or people in couples looking for secondaries. I'm in a couple of secondary relationships, but as I said in my OP, it doesn't change my feeling of being single. It doesn't change the fact that I am seeking a primary partner (I honestly wouldn't mind getting married, and would love to have the option of raising a family) and poly communities tend to be the least likely place to be able to find one because they are dominated by couples already in primary partnerships. I went to some different poly discussion groups and ended up giving up on them. When bringing my situation to discussion groups, I rarely get feedback that indicates any awareness or sensitivity to my journey as someone who is un-partnered. And in many cases, there were people considered me "just a swinger" and would say just as much.
I think a lot of these responses are just due to the fact that the pervading culture in these poly communities is a culture of couples. Many of these people have never approached the world of poly without a partner already and so they simply don't get it from a single person's point of view. And unfortunately, they rarely leave room for it to be discussed. (even this thread had to be redirected back from a discussion about triads, etc.) So most of the poly-oriented singles I know pretty much stay out of local poly communities because it just ain't worth it.
Many of us have just decided to drop the label of poly altogether because the pervading culture just doesn't seem to apply to us. Labels are a pain anyway, as the whole collision of definitions over "single" can illustrate.
I've never called myself poly anyway. I consider myself open to a variety of relationship styles and I know that monogamy isn't my path for that. But I'm a very community oriented person and it's a shame that it's so difficult to find a community that has room for this very important part of my experience.
Hmmm...maybe the self-identified poly (or open) singles should holla and we could form our own community.