From my perspective, a large part of the fear and broken-heartedness in all kinds of relationships is due to insecurity, jealousy, not doing your own emotional work, etc. And don’t get me started on the historical/anthropolgical fear of women’s power and women’s sexuality, which is also a huge factor in many of the examples you cited.
We are constantly bombarded with messaging that indicates that we should only love one person, and that makes it difficult to know how to go about doing anything else and so we (as a society) go from one broken relationship to the next, more wedded to the idea of monogamy that actual commitment to any one person. Polygamy doesn’t ultimately work well because there is such a power imbalance; polyamory doesn’t always work well because not everyone does their own work to be healthy individuals.
Yesterday I wrote about What I’ve Learned About Love From Non-monogamy. When you stop looking at love as pie to be divvied up and instead as an infinite resource it becomes incredibly easy to love more than one person. I have another life partner, but my primary partner and I have both recently fallen in love with the same woman. This all works because we are not trying to control or dictate the lives of either one of them. I think this is the real issue with love and relationships, not the number of people involved.