1. Why do you believe what you believe? What would change your mind?
Because I have actually seen what poly communities look like and treat that as my main evidence rather than idle speculation about human psychology and abstract social norms.
Poly communities, basically invariably, put an onus on their members of perpetual sexual and romantic availability and desirability, and jeopardise the standing of anyone who can’t maintain those. It’s not hard to produce explanations for these trends, of course, but there’s no need to rest my confidence in them on conjectural a priori reasoning: they are visible, empirically, without need to defer to just so stories. Consistently, members who are exceptionally able and willing to engage with many partners find themselves with positions of correspondingly exceptional social status and power, while those for any number of reasons unable to do so (parents and non-twentysomethings, for example—point me to a single poly community with that’s not at least 80% childfree or under 35) find themselves at the margins and worse.
The point about older members (including parents) bears stressing: if you treat anyone middle-aged in your local social graph as creepy-until-proven-innocent, and reliably fail to make community events and spaces appropriate for children, your “community” is failing at an essential task. The healthiest, most attractive, and least burdened age range in a population is the one that will (on average) require communal support least, and yet it’s consistently the demographic poly-dominated social circles mainly cater to. A community is, first and foremost, where you live your life, and if that excludes both the first eighteen and the latter half of its years you’re in a very bad way. (This policy of graduating members into communal abandonment renders pretty darkly ironic the common argument that polyamory provides an alternative to monogamous social atomisation.)
Of course, you might say this is only a criticism of basing communities around polyamory, not the relationship style on its own. Maybe, you’ll say, it’s a bad idea to turn poly dating scenes into all-encompassing communities, but the same is true of birdwatching and you don’t see me calling for stores to toss their field guides and binoculars in the interests of social harmony.
To which I respond: I’ll believe it when I see it. Poly people cluster together, and it’s not hard to understand why: relationships and sex are a large part of most people’s lives, and it makes sense they would seek out social networks where the way they practise them won’t stick out as an oddity. Of course not every poly-heavy community forms around polyamory as such, which is a problem on its own; often you’ll find yourself stuck with a poly social circle just because you’re kinky or trans or think AlphaGo gaining sentience is a bigger global risk than climate change, meaning if you’re mono you’ll be stuck choosing between social disconnect, romantic loneliness, or dating in a way you find gross and incomprehensible. Which you’ll find ready examples of if you scratch an inch below the surface: plenty of people in the demographics I just mentioned will talk about the awkward choice of either a dating style they can’t stand or strained social relations, either over lifestyle with normies in a mono-by-default community or over dating style with the people otherwise like them in their poly community.
Of course, since this (unlike most defences of polyamory and a few too many criticisms) all rests on empirical observations about actually existing poly communities, it’s also open to empirical refutation. But after a certain point alleged empirical counterexamples in the form of poly communities with healthy norms become more likely than not to be noise, and if I have not reached that point I have come very close.
2. A monogamous person has a crush on someone other than their partner. In a healthy relationship, what would happen next?
These sorts of scenarios are wheeled very frequently as cases that polyamory handles more smoothly than monogamy, which I think speaks badly to the self-control of most polyamorous people. Everyone experiences crushes that it would be inappropriate to act on: towards people whom they know wouldn’t reciprocate, for example; or too young for them; or in a professional setting where consent becomes iffy. And if you haven’t figured out how to tone these feelings down yet ditching monogamy won’t do anything but kick the maturity can down the road.
(Of course, it speaks to this culture of indulgence that poly communities tend to be especially weak on these points, cf. the feigned incomprehension at the stigma on age gaps you frequently find in them, or the rate at which they encourage solving work/life balance problems by straining the boundary between job and polycule.)
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 is a weird counterfactual to assess: am I supposed to imagine the actual population suddenly switching en masse to open relationships? or am I supposed to imagine a different population where polyamorous dating norms would be remotely stable?
If the former, the answer is easy: there would be an immediate spike in discontent and people would overall either explicitly or de facto switch back to monogamy and look back on their brief societal dalliance with free love as an embarrassing act of collective naïveté. Most people simply aren’t constitutionally suited to polyamory, witness the striking rates of weirdness you see in any actually existing polyamorous community and the way that among vanilla normie monogamous couples“opening up your marriage” is universally understood as a prelude to an especially nasty breakup/divorce.
I don’t think the other question is very hard, because we essentially have a long run of test cases in the form of every queer, kink, or rationality community anywhere: people would have fun elevating cuddle parties, hippie college dorm culture, and hookup scenes into the governing paradigms of social life from their late teens to early thirties before settling awkwardly into isolation in middle age. The difference being, of course, that rather than containing this failure mode to a few fringe subcultures it would become essentially inescapable.