1. Why do you believe what you believe? What would change your mind?
I believe what I believe due to a mix of direct personal experience, secondhand accounts from people in my broad social circle, and theoretical this-makes-sense-ness.
I’m polyamorous, and this has led to me being far happier than monogamy would have forced me to be, with no negative externalities on society that I’ve noticed. My girlfriend, my plausible future girlfriend, and various other friends of mine are similarly happy-without-negative-externality as a result of being polyamorous, to the best of my knowledge. And then, on a pure theoretical basis, I see no reason to expect things to be different for people further afield from my personal social circles; fundamentally, there’s just no obvious-to-me reason to think that letting people date more than one person at once, or be married to more than one person at once, should go any worse than letting people be friends with more than one person at once, which is widely acknowledged to be not only unobjectionable but actively good.
It’s very unlikely that anyone would be able to change my mind about my personal positive experiences with polyamory; but my mind could be changed about other people’s positive experiences with it if they were to report that actually I’d been misreading them and their experiences were in fact negative all along (although I don’t at all expect that to happen), and my mind could be changed about the lack of negative externalities if people could point out the presence of any such externalities. (Ideally not ones which come down more to personal not-getting-along than to anything inherent to polyamory, as seems to be common in attempts to do so.) And, of course, I’m open to theoretical arguments about why I actually should expect it to go badly, although none that I’ve encountered so far have seemed at all plausible to me.
2. 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?
The person goes to support their secondary partner, with all parties involved being understanding, in the same way that I’d expect people to be pretty understanding if someone in a married couple had to cancel a date in order to visit a close friend who’d just landed in the hospital, except moreso.
Or, alternatively, the person goes to their date with their primary partner, since keeping up that relationship is important to them, but they make sure that someone else is there to take care of their secondary partner until the date is over and they can come themselves; because, after all, one of the great virtues of polyamory is that one doesn’t have to be stuck with just a single partner as one’s sole source of partnerly support.
Or some other thing entirely, although I’m going to stop with the examples after two. People are sufficiently varied that there’s no single Obvious Right Thing To Do which applies to all trios of people-in-healthy-relationships-of-the-described-configuration; ultimately, the right thing to do is dependent on the people involved and the details of the relationships between them.
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.)
Well, probably some obligate-monogamous people would be pretty miserable, at least if my naive guess is right that they comprise more than 10% of the population. (I’m not aware of any rigorous statistics on the matter, although I’d be interested to see any that in fact exist.) Excessive social pressure against monogamy would be bad for all the same reasons that excessive social pressure for it currently is bad.
But, if we assume that it’s more “everyone is polyamorous except for those who specifically prefer to be otherwise”, with the 90% number a shorthand? I’d expect people to be happier, for all the same reasons that reducing societal restrictiveness makes people happier in general. More options will be open to them, relationship-wise. Huge numbers of happy relationships will get to form which, against the backdrop of modern-America-style social pressure towards monogamy, would have stayed unformed. People will get to bask in the happy feeling of their partners living happier lives, and will get to bestow the same feeling back onto their partners. Child-rearing will be less of a horribly exhausting activity, due to the likely presence of extra people able to pick up the slack.
On the flip side, a lot of the negative effects of modern-America-style restrictiveness will vanish, or at least drastically reduce. There won’t be any need to worry about what to do if one gets a crush on someone other than one’s partner, or about exactly where the boundaries of what does or doesn’t count as an overly-close relationship lie. People won’t need to be anxious about their inability to fill every possible want their partner might have from a relationship, if their partner dating them doesn’t prevent their partner from also dating other people who fill other wants. Et cetera.
On a separate front, I’d expect society to end up forming some sort of new Default Relationship Model, rather than taking on the “negotiate your own details” approach currently common in polyamory, given the thing where many people like having easy relationship defaults to fall back on. (Conversely, I’d expect the remaining monogamous people to become more detail-negotiation-inclined, since their preferences would no longer be compatible with the society-wide default.) The new default might or might not end up becoming as restrictively enforced as the current default is; I’d hope it wouldn’t, but even if it did (which I can’t at all write off as a possiblity), it would be restricting people into a less-restrictive default rather than a more-restrictive default, which would still be a net improvement over the current status quo, even if one with plenty of room for further improvement of its own.