1. Why do you believe what you believe? What would change your mind?

Bringing up children is a core task of a family unit. I would go so far as advocating that this is the sole rational foundation for the legal privileges usually granted upon marriage: the state has no reason to favor one relationship type over another just because the people involved have (or are thought to have) sex with each other. I believe that current secular norms regarding “no-fault” divorce or the non-stigmatization of non-marital cohabitation have (in spite of their proponents earnest desire to prevent tragic situations) set in motion (or at least greatly increased) social changes that ultimately resulted in the increase of the prevalence of children being raised in single-adult households, which naturally burden the “single-adult” and are strongly correlated with increased poverty and with the lack of opportunity for parent-child interaction and the child’s intellectual and social development.

From my limited readings, I got the impression that polyamorous relationships entail a very fluid understanding of the obligations of one member versus the other(s), going as far as requiring one member to not merely tolerate but to positively cherish the others’ outside romantic attachments as one important part of the partner’s “self-actualization”. I cannot see how this emphasis can be harmoniously combined with the ensuing likelihood of looking at each of one’s present attachments as intrinsically temporary , and I believe this is likely to result in much less stable arrangements and a less well-defined set of obligations towards the child.

I would change my opinion if, in a carefuly controlled study (or as controlled as feasible in the social sciences), polyamorous relationships were shown, across all social classes (though especially in lower SES) to:
A) not increase average, median or first-decile child poverty, <18 yo delinquency rate
B) not decrease children’s average, median or first-decile academic scores and mental health/social adjustment scores

2. 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?

A loving and frank conversation should ensue to understand why one partner fears abandonment or why the other is tempted to look elsewhere for the fulfilment of their romantic needs. Ideally, both partners should strive to understand the source of their feelings and work on improving mutual displays of affection/care.

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 think that the loosening of sexual mores has made it more difficult for people with less sex-drive or who (due to philosophical/religious reasons) value chastity to compete for partners. This also has consequences in people (mostly women, due to their average lower libido) , whose interests in “going slowly” are now no longer as culturally supported , and who now may feel more pressure to engage in a sexual relationship before they really feel comfortable with that step. Likewise, I expect that, if 90% of the population became more comfortable with the idea of non-exclusivity, the remaining 10%, as well as the “bottom-half” of desirability distribution will become more handicapped in their quest for romantic fulfilment due to “prime mates” no longer having to (overtly) “settle” for one partner. Less extrovert/”sex-appealing”/etc. men and women will find themselves socially pressured to accept a “lesser role” compared to their partners’ more numerous relationships, and this may cause resentment. I do not claim that the total amount of romantic/sexual resentment will be larger than that in our present society, but I do believe that the resentment will be “re-distributed” in ways that do not strike me as socially advantageous. Nowadays, introverts/high conscientiousness individuals (both men and women) who are willing to devote themselves to a stable relationship for the sake of a larger social purpose (supporting their children and partner through thick and thin, for example) do have some expectation (when in a stable relationship) of being “rewarded” with increased romantic/sexual opportunities. If they are, instead, placed in a position where, in spite of their pro-social (but not “sexy”) qualities, their partners are encouraged/expected to spread their love around, their incentives will change and they will likely feel exploited and feel that society, as a whole, does not care about their needs and that pro-sociality is a fool’s game.