Real intimacy requires investment. Relationship anarchy, any time I've seen it attested or practiced, faciliates the opposite. It's a fetishisation of alienation. What you're describing as 'pressure of expectations' can be understood very differently, as the expectation of reciprocity. In other words, being able to rely on people - whether as friends or lovers, when things get difficult. Without that, all we have is limerence and capriciousness.
I say all this as someone who's been in non-monogamous relationships of various kinds - from weeks to years. Without the possibility of commitment and the acknowledgement that all relationships are inherently hierarchical, we atomise individual needs and make real enduring connection and community impossible.
I had much less direct and indirect experience, but I know what you talk about. Even more, it is also my experience that these constellations are unstable. But I see this as them existing in a sea of monogamist society, surrounded by prejudice and contempt. Try to introduce this to friends and family. It's similar of how gay relationships are much more often open, due to (guess:) societal context like a patriarchal society.
Historically disconnected societies were used to be more creative. I hope they would also be in the future.
To your second point - sure polyamorous relationships are countercultural, and this inevitably puts more pressure on them from family etc. However they're also innately more complex, and require far more processing than conventional relationships. They'll always be a minority for this reason alone. And that's totally fine. Your relationship style is no less valid for being less popular. This need to proslethise to others is itself unhealthy. Tolerance is important, uniformity is not.
All of the "classical sins" could be described as are human nature regardless of relationship type. Note that as an alternative I don't just think of flat hierarchy polyamory here, any non-heteronormative relationship or constellations that revolve around a "main relationship".
In a world of industry standards, automatization, light and flexible work hours, easy communication and high mobility I think we need to be creative again. Especially how this above evolution seems to reach its limits, and "scale to the moon" does not seem to work (dead internet) and we're in dire need of small, informal communities again. I see much more openness to this from the younger western generation.
For everybody else, there is the normal and perfectly human feelings of jealousy, attachment, fear or loss, and feeling associated with self-confidence.
1) Does not prioritize you
2) Finds somebody they like more than you
3) Not actually happy with you but still uses you
4) Is going to get STDs from other people
5) Will have less and less time for you because of others
6) Believes children can be raised "by a village" instead of their own hard work
7) Wants to involve other people in your life
8) Births a child with somebody else (maybe?) as the parent
9) The mere thought of them with another person grosses you out
A group of people sleeping together is not a stable community. It's filled with people who are trying to sleep with other people inside and outside of the group who are vocal about being able to spend time, money, and effort on others for sex. There's nothing binding a group like this together besides sex.
Even normal community activities like volunteering or sports clubs have drama and people who end up hating each other. Add sex in the mix and you've created an explosive dynamic.
https://davidgraeber.org/articles/are-you-an-anarchist-the-a...