1. Why do you believe what you believe? What would change your mind?
Polyamory gives people more choices and more freedom in their relationships. People are different – they have different strengths and skills, and that includes in interpersonal relations. On the flip side, people have different weaknesses, and those weaknesses become all the more apparent in an exclusively monogamous relationships. Someone may be a wonderful partner in most ways, but if they struggle to engage with their partner in a particular way – for example: intellectually, or emotionally, or sexually – both partners are harmed and a strain is put on the relationship.
Polyamory allows people to be more fulfilled in their needs, which when done right will strengthen all their relationships (romantic and platonic). Living in larger communities than a traditional nuclear family is healthier too – there is a greater support network to ensure everyone’s needs are met.
A polyamory norm in society doesn’t prevent people from choosing monogamous relationships, but a monogamous norm does discourage and stigmatize those who would seek and benefit from polyamory.
To change my mind I would want to see evidence that polyamory is on net harmful to society – that people are significantly more hurt than are fulfilled. I would change my mind if I saw that widespread polyamory led to greater breakdown in relationships, rather than a strengthening.
2. A polyamorous person hates their partner’s other partner (their metamour). In a healthy relationship, what would happen next?
The key to any healthy relationship is communication. Both partners should seek to understand the other’s perspective on the third person. This should lead to healthy and appropriate boundaries in the relationship.
3. What would happen if 90% of people in a society were polyamorous? (You may assume they all practice one style of polyamory, or different styles.)
I would expect a polyamorous society to have stabler relationships as people feel more fulfilled, with less points of friction in stable relationships. This would have a knock-on effect – stabler relationships would have positive benefits across the board: people with a stronger support system have better mental health outcomes, can better recover from negative life events, and raise healthier children. It would be a small effect size, but broadly applied net positive.