This is just my personal perspective, experience on the topic:
I've been labeled as such multiple times before, had some experiences with this and just about a year ago I've been added to some online groups, even visited some meetings also - although currently I do not have partner/relationship, I specifically decided not to label/pre-define anything or anyone - regardless of poli or mono, because as I see - it's rather saying the most about myself and my own relationship with concepts and definitions anyway.
As member of the Desteni group most here are probably more aware of the fact that becoming reactive, judgmental and thought/emotion-possessed can happen with any human pretty easily, especially if the foundation with self is not under 'scrutiny' - and then it's pretty easy to slip away for a longer 'drama' with partner, regardless of how many people are involved, being influenced, etc.
In a really clinical view, all who are under the same roof, like children, grandpa, I am with relationship, of course not 'amorous', but as interacting, communicating, sharing.
I've been with partners who were in 'open relationship' and mostly I was able to see some points they could work within self-honesty in terms of responsibility/blame/projection and I sometimes reacted to those, sometimes not.
Within the poliamorous/open sex groups what seemed to me as primal is that these people somehow concluded that they should and even need to be open for more partner at the same time. They seem to trust their gut about when they should open up to another, and if they still find 'value' within the current one, they rather not sneak around, like you said as 'cheating', but try to keep both parties at hand.
There is a term 'unicorn' there, for instance a single woman who would 'join' to a couple with 'full and open' love to both person and then this perfect threesome love blossoms to the greatest joy in the universe. However, that's why it's referred as 'unicorn' as it's so rare, that it's better not to even seek. Well, love, as a definition, also something one should investigate and purify, but mostly as comes goes when funded by energy of the mind, so if one invests into this, uses as refugee and compass for relationship, partnership, then probably will have quite a ride of ups and downs totally powerlessly. Not always, maybe the ones 'working well' in the world system are specific placements to balance out, make others crave and thus create more friction in their minds, I am just guessing here, but I have seen multiple partners being open with each other in all ways apparently.
Do they 'perfect' the first, then engage/open to another? Not sure. Do they completely aligned/agreed with their already existing partner/partners in pool about what they want/will do? Not always, but there is such as well. Not saying beyond Desteni everyone is screwed, but most humans are complete slaves of their mind/desires/personality/fears, thus when we engage in the world, mostly interact with people never met the concept and practical application of self-honesty, and that probably means some degree of self-interest, sometimes packaged with good intentions, sometimes just expressed more purely.
From my perspective, what started with me opening up to this concept was that with my partner there were like 2 of 5 things what was cool, but the other 3 aspects I could not 'live out' I wanted with her - and she was pretty denial on those(not sex-related), such as going wild trance parties, do some stuff she just did not like, living in village/farm area, etc - what I was able to meet with another women who were really open to and the idea occurred to me that if I could establish a really trusted agreement with her, here meaning - I will not leave her, will not do sex with another one for example, then why not to 'go out' and just express myself, have fun, explore, expand and connect?
My partner was pretty jealous and she had the impression I just wanted to f*** everyone and even it was not the case, this was influential to he utmost for our partnership.
There was sexual desire/fantasies also behind these, but in a way I was able to see that those points within me is self-dishonest/self-interest and when my partner eventually got attached to another guy and she wanted to bring 'him in', I've felt cheated and threw her out and then eventually she came back when she left that guy but after all this was quite severed already and then from that point she had someone else from time to time and for me it was not a huge problem, just when she became quite maniac with her jealousy while I did do anything with somebody else, then it started to be not really supportive anymore as there was no respect, trust, integrity, thus I left and since then in theory this might seem like a great fun, I am quite cautious about these, as I've already burned myself once at the fire, so why do it again? And I'd rather say it's not much about fearing it, but common sense.
I do not think anyone would want more than one woman(or man) if would really stand into a fully committed agreement, as it's quite a job already(still not about sex, but for support, process, purpose), but if I ask honestly the question and also answering honestly for myself that "Is this what I want?", even for a period of time, to see what this brings, where I stand, do I commit myself to transcend anything of thoughts/feelings/emotions coming up on the way, and if it's alright for me that my partner might not stand to this as me, and it's really okay for me that she/he will go out and have fun and even having the chance that she will just 'stray away', and I still stand, unwavering, firm, no blame, no jealousy - and probably for a lot more questions like this and I want this(good luck), then why not?
This is our life, the only rules should matter is do onto others what you would like to receive and investigate all things and keep what is best(for self - and all).
But when I considered then, I realized, well, this is then not really a relationship with the person, but rather mutual interests meet, nothing 'serious'.
In a way this also can be a 'tool' to investigate my patterns of 'wanting to own' my partner, or not to be 'endangered by' being owned by my partner, thus 'working on my ego', but within self-honestly one can realize that there is a superego behind all of this anyway, meaning, a conviction, a fear from something, possessed by an image, an idea, a desire - or to just hide within multiple superficial relationships too...
There are books exploring pre-civilized era what supposed to say that more open sex and relationship was the normal within tribal, pack-like lifestyles, in the old, middle ages, even there is a community around, who I met recently, they say that monogamous partnership is an aberration, result of a bigot and twisted brainwashing and enslavement and they do loudly promote the really open partnership, but that's also the polarity extreme of the whole relationship-definition-spectrum, I guess...
I've also explored quite some alternate communities, 'hippies' too, when I was travelling around and there are places, where are no rules at all, but when someone is reactive, there is drama and mostly people can't handle that, thus it's leaving an uncool taste for all everyone around them too.
Or a matriarchal tribe in a city, wherein the mother of the children is the center of the family, and she might had multiple men before, and she 'keeps their children' and always the man she is with is responsible for supporting her and the children, regardless of whose kids they are. Interesting concept too.
To choose only one partner based on any fear also does not seem as self-honest, but as I was walking though DIP Agreement Course, I had to(and wanted to) explore why would need ANY partner at all, and then why would need ONE too.
To be able to word(name the game) what I do NOT want too also can assist to avoid relationships, what I realized that "This is not what I really want or would not support process of self-realization - me".
And of course it's always great to review 'what I really want' and is it really-really self-honest or there is any conviction/definition/desire/fear I identified myself with and wanting to 'live out'.
And as you mentioned, it's really easy to have this un-worded mutual agreement within these kind of relationships, that 'we just have fun', quite superficial relationships, one night stands, and then people are unprepared, they do not study, know their own mind, personality, and then they are basically being run by their convictions, regardless of those are aligned with what's best for all or not - and then all of a sudden someone feels attraction to another and then they can 'feel the love' - and then it fades, or they build more on that and they marry.
Often people 'got their heart broken', so then they do not get vulnerable, open, really intimate, even if they say, act like, do so, within self probably not, and not even admitting to the fact that they fear to be owned by their own mind, but the fact is that they already are...
Society definitely can judge, even in some countries people are being killed for this, so it's often being kept this in secret, or sometimes they 'come out' when a woman I know of, she walked out with two man holding her hands and it felt really good for her as she described.
Kids usually do not make big thing about it, but they are still in the programming phase in a way.
For me, when I had a partner who was also with someone else sometimes, after a while I felt more 'space' within myself to grow about this, to spend more time together, taking responsibility for both if possible, and it's obvious when the other is open for that or not.
Then I was feeling this as it's becoming 'not enough' for me, and after we discussed, I realized well, this was big fun, but I have to consider letting her go and probably not because she would sleep with another guys sometimes, but because maybe I am realizing this is not what I want. Then again: why? What's the best for all participants here, etc...
I think DIP is extremely supportive here, including first and foremost the buddy, who can have our 'third eye' within this, wherein I am 'invested' already, thus having a relationship what can influence my view, understanding to the degree of not seeing, realizing absolute self-honesty within a situation.
My personal experience was also that I have this idea of a 'perfect partner', who I would really want to have with me for life, but that never comes, so then I started to decompose what that means and realized, one really funny, clown cute girl I really like, but I can't take her seriously, another one was really wise and smart, third was a professional dancer-massaguer, the fourth one was really great with kids, so I was wondering about having a harem, but they did not seem to like the idea that they all would dedicate their life within each other's total acceptance for me, lol, obviously, so it was busted quite easily.
Well, then the idea came, why not they also would bring some guys to the table and then live in a 'small community', like 6-8 people, and then we really take honesty, respect and openness seriously, but that still never came and I also realized that although my mind can keep telling me ideas and fantasies, when I take myself to actual opportunities, I am there and in self-honesty it's not really what I would like to do/have/be, so then I was able to let go these desires.
In a way I do not follow morality, or any other principles than be self-honest and I am always equal and one within what I find myself and how to take responsibility to my utmost potential and considering to give what I'd like to receive, so I do not hold myself back based on society-rules, but also I can debunk(and I trust myself that I am going to) any crazy desires I can experience.
Some desire is so silly that I can just say: haha, nope. But some can be more subtle, then reasons and justifications are backing them up and slowly building up to conviction/self-definition and then I have to check, is it cool to go and explore that is it really what I want - then yes - I check it out, and mostly it's some self-dishonesty I find 'out there' to be reflected back to my starting point - well, but this is the same regardless of how many partner one would want.
I close this by realizing the most supportive point within all - is to see what is the things I would expect from any(amount of) partner to experience, live, gain, and what is the quality I do not live by myself by projecting out these to somebody and why do I limit myself like that?
For my case was like 'funny', 'beauty', 'smart', 'wise', attractive', gentle', 'sweet', 'cute' - and to investigate my relationship/past/memory/reactions/expectations/judgments to these words supported me to not rely to partner to be whole of myself here.
Yet I still can find craving within, but mostly I can assess - is this 'desire' based on a frustration to compensate, distract from - or I am genuinely finding this particular person enjoyable and supportive too?