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.
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
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.