1. Why do you believe what you believe? What would change your mind?
Monogamy has always been an important feature of Western Civilization, and many other civilizations besides. All relationships are difficult, but monogamous people have Tolstoy for the hard times and Shakespeare for the good: long tradition both proves the model and teaches from experience. Meanwhile poly people have Sex at Dawn and The Ethical Slut, plus a lot of confident online commenters. Monogamous people go through ups and downs, and sometimes bitter break ups, but over time they grow together, they raise children, and they teach their children how to have relationships like theirs. Thus society continues from generation to generation. Poly people tend to remain suck in childhood themselves, their lives filled with petty drama and haunted by unacknowledged uncomfortable feelings. The institutions of civil society are overwhelmingly built and maintained by monogamous people for the benefit of themselves, their children, and their communities; poly people may be active within a subculture and occasionally write good software but are often disconnected from society at large.
If polyamory were to become more widespread, and society were to continue to flourish with it, I might be convinced that it is not necessarily worse for society. If I saw poly people living full lives, rather than skating along on the margins, I could be convinced that it works for individuals.
2. A polyamorous person hates their partner’s other partner (their metamour). What typically happens next?
In situations like this, people generally agree to a “compromise” that reflects the underlying power dynamics of the relationship. If the partner in the center of the triad is more attractive or has greater social capital than the person who hates the metamour, they will use their superior bargaining position to get what they want. The unhappy partner may “acknowledge the need to unlearn their toxicity” or something, which just means acquiescing to the consequences of their weak position.
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.)
Many societies in history have been 90% polygynous, if you count the guys who remain single because the women are all taken. They have been highly unequal and often unstable. If a society such as ours somehow became 90% polyamorous, in any of the styles commonly practiced by readers of this blog, their low fertility rates would likely mean that polyamory were not so popular in the next generation.