1. Why do you believe what you believe? What would change your mind?
Polyamory is clearly not worse for its participants than monogamy: if it were, why would they choose it? Unlike with monogamy, virtually no poly people are modeling their relationships after their parents; rather, it’s a conscious choice typically made with significant research and counsel. Many poly people have had bad experiences with monogamy (including e.g. one parent staying with an abusive spouse), and could not thrive in a monogamous relationship. Mostly though, I believe what I see: the poly people in my life are generally happier, more stable, and better in touch with their own romantic needs and desires than are my monogamous friends.
That polyamory is not worse for society need not be demonstrated to support my beliefs on the question as stated, so suffice to say that it would be hard to match the litany of suffering monogamy has to its credit.
I suppose I might change my mind if I saw large numbers of poly people becoming monogamous, and describing how the change improved their lives.
2. A polyamorous person hates their partner’s other partner (their metamour). In a healthy relationship, what would happen next?
Talk about it! Clear communication is Poly 101, and these kinds of situations illustrate why it’s so important. Where does the hatred come from? Is it jealousy? Fear of being replaced? Comparison and self-esteem issues? These are all perfectly normal feelings, and they can all typically be resolved by communicating clearly, especially within a supportive poly community. Working through situations like this together can bring partners closer together, and the connections so forged are one of the most rewarding aspects of polyamory.
3. What would happen if 90% of people in a society were polyamorous?
There are examples of subcultures that are majority polyamorous: rationalists, the kink scene (in some cities), some social justice circles. In general people are more aware of and open about their needs and desires, and more willing to interrogate their own attitudes. Poly relationships can be a lot of work, so a 90% poly society would probably only be stable where resources and technology allowed people sufficient time to maintain multiple healthy relationships. Realistically though, polyamory works best as an opt-in relationship style, so poly people in a majority poly society might not get all of the benefits.