1. Why do you believe what you believe? What would change your mind?
In general I think the onus of proof is on showing some activity is unacceptable rather than on showing that it’s acceptable, and nobody has ever given me convincing reasons to think dating more than one person falls into the former category rather than the latter.
There are, of course, any number of unconvincing reasons that get trotted out. It’s claimed that it will inevitably lead to destructive jealousy, or that human beings are psychologically incapable of being in love with more than one person at a time, or that it incentivises dating market structures that force widespread dissatisfaction. But these claims, on examination, either fit poorly with empirical data or seem to draw exclusively from cases of formerly monogamous couples “opening up” their relationships to disastrous consequences rather than being openly polyamorous from the outset. My actual experience of poly people accords very poorly with these usual allegations against polyamory as a concept—and so, on the heuristic that of good arguments against a widely known position have yet to be discovered they probably don’t exist, I accept that there is nothing impermissible in dating more than one person at a time.
I don’t mean to suggest I have no positive reasons for thinking polyamory is good: it helpfully resolves notoriously intractable questions about “what counts as cheating” in a very straightforward way (nothing does), it allows for larger family units and alloparenting without requiring reliance on families of origin, it removes a common source of anxiety about partner abandonment (since their falling in love with another person no longer forces in principle a choice between you and your rival), and it offers a range of distinctive pleasant experiences otherwise impossible (helping your boyfriend with his dating profile, polycule cuddle piles, etc.).
I always find “what would change your mind” questions puzzling, just because my usual experience of discarding old convictions is not of witnessing evidence I specifically predicted would not exist, but in general I would be willing to revise my views if someone could offer non-speculative evidence polyamory tended to diminish human happiness, either among its practitioners or in society at large.
2. A polyamorous person hates their partner’s other partner (their metamour); in a healthy relationship, what would happen next?
This clearly depends very heavily on the metamour and the basis for the hatred.
This becomes obvious if you ask the same question about a friend of your partner’s. Do you hate them because they hum incessantly and uncontrollably in a way you find intolerably grating? Then invest in some ear plugs or noise canceling headphones, or just politely vacate yourself when they come to hang out. Do you hate them because they’re of a race or culture you despise? Then learn not to be a bigot. Do you hate them because they try and control things like when your partner can eat or sleep and who they can hang out with? Then [insert advice about helping someone who is being abused]. Romantic relationships are human relationships and don’t exist apart as some sui generis association with its own inner logic and norms.
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.)
This feels like a worldbuilding prompt, and worldbuilding tends to bifurcate into perfunctory snippets and encyclopedias, so given space constraints against wikis this will be short.
It’s hard for me to imagine such a society not being massively less lonely than our currently existing one. Polyamory is an especially robust guard against isolated (fractured) nuclear families as the standard living unit, and those go along under almost all circumstances with great strain during childrearing and impoverished social lives more generally. Such denuclearisation would provide more windows looking in to serve as checks against abuse, and more avenues for escape from particular toxic relationships than the alternative. One suspects, indeed, that these are among the very reasons many people react to the thought of such a society with horror.