1. Why do you believe what you believe? What would change your mind?
I believe that polyamory is good because relationships are good. Sex is good. Mutual emotional support is good. Dinner for two at Luigi’s is good. Meaningful discussions about the semiotics of Ginny Weasley Potter are good. Visiting your partner in hospital (or prison) is good. Assuming that A’s relationship with C doesn’t detract from their relationship with B (which absent irrational jealousy or unmanaged conflict between metamours it shouldn’t), polyamory allows more of all of these things than monogamy.
Furthermore, I believe that the vast majority of people find it easier manage jealousy in a poly relationship than to manage the fact that a monogamous partner is only meeting some of their relationship needs. A monogamous relationship requires A to meet all of B’s relationship needs, and B to meet all of A’s. Finding this complete double co-incidence of requirements (a so-called “soul-mate”) is a rare stroke of good luck. Under polyamory, the requirement is that both paramours meet at least some of the other’s relationship needs, and that they are able to tolerate any metamours. This is a much lower bar.
The main thing that would change my mind is evidence that most people can’t manage jealousy well enough to be happy in a poly relationship. To get hard evidence you would need a rigorous way of measuring proneness-to-jealousy, and a study showing that it was resistant to change (probably that it didn’t shift after someone formerly monogamous spent several years in a poly relationship). There is quite a high bar here, because the divorce rate is proof that many people can’t make monogamy work either.
2a. A monogamous person is jealous of their partner (for example, because they’re afraid their partner has a romantic interest in someone else). In a healthy relationship, what would happen next?
The couple would discuss what is making the jealous partner feel jealous and how it is affecting their relationship, and then agree either that the jealousy is unfounded, or what steps are needed to restore trust and eliminate it.
2b. A monogamous person has a crush on someone other than their partner. In a healthy relationship, what would happen next?
Ideally nothing, but realistically most people can’t hide their crushes if they see the object of the crush often enough, so eventually this is going to turn into a justified jealousy case (and given the norms of actually existing monogamous culture, the couple won’t be able to communicate about the jealousy in a healthy way). Of course there is also the possibility that someone leaves their current partner for the crush, which is a disaster if there are children or other serious commitments involved.
2c. A polyamorous person gets an STI. In a healthy relationship, what would happen next?
Ideally the STI is caught by routine regular testing before it can spread at all, or at least while it is still possible to trace contacts and test everyone in the polycule. This is far more likely to happen under polyamory than under monogamy-with-cheating because polyamory removes the stigma against taking a STD test or asking your long-standing partner to take one.
2d. A polyamorous person hates their partner’s other partner (their metamour). In a healthy relationship, what would happen next?
The three people involved talk about it like grown-ups. If there is something about the shared paramour’s behaviour that is making the situation between the matamours worse, then it may be possible to change that. Alternatively it may be possible to negotiate a split of the shared paramour’s time such that the metamour’s don’t need to come into contact with each other. As a last resort, one of the relationships might need to end.
2e. A polyamorous person has a date scheduled with their primary partner, but their secondary partner is in the hospital with an emergency and needs support. In a healthy relationship, what would happen next?
Obviously the partner in hospital needs the paramour more – this isn’t difficult. Depending on the relationship between metamours, either the shared partner or the primary couple visit the secondary in hospital. The date is rescheduled.
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.)
A lot of the problems with polyamory as it exists now would go away if it was the default. Most people would acquire the necessary skills to manage poly relationships in high school. Over time society would co-ordinate on sensible default arrangements for how to handle e.g. finances when a partner enters or leaves a polycule. Widespread existence of more-than-two-parent households would lead to stronger alloparenting norms more broadly, which is a good thing in its own right for the obvious practical reasons.
Overall, there would be less sexual repression (in high-conscientiousness subcultures) or jealously-driven conflict (in low-conscientiousness subcultures) because A’s interest in C and C’s interest in A cease to be a threat to A’s pre-existing relationship with B. More people would be having more relationship needs met more consistently. For Great Justice!!!