1. Why do you believe what you believe? What would change your mind
Family is what people depend on and monogamy strengthens the bonds of family. People like to talk about choosing your own family. Sadly this really does not work. The fundamental characteristic of being family is that you cannot stop being family. When people need help they go to their actual family for aid. Friends drift apart but family usually stays together. This is not to say family is never abusive and never lets people down. But your family is your best bet when you need help.
Committed monogamy is the only real way to bind people as family. There is a reason marriage vows talk about staying together in sickness and in health. Not everyone upholds their marriage vows but many people do. Very few people are going to take care of their polyamorous partners for life when they develop a chronic illness. Once you have children, it is even worse if a relationship breaks up. The benefits of polyamory are small compared to the cost of making your family much less stable.
There are many reasons why poly appeals to the rationalist community. Rationalists are unusually likely to be LBGT. Many of them have been treated badly by their own family. Few rationalists even want children. Polyamory has led to quite a lot of drama in the rationalist community but it makes sense that the rationalists tried poly. But most people are in a completely different situation.
I would change my mind if I saw evidence that poly relationships were as stable as monogamous relationships. It is also possible I am underestimating the benefits people derive from polyamory. Mostly the benefits seem to fall under ‘fun’. There are much safer ways to have fun.
2a. A monogamous person is jealous of their partner (for example, because they’re afraid their partner has a romantic interest in someone else).What typically happens next?
A healthy monogamous relationship is a team. Whatever is good for the team should make both people happy. You should be rooting for your partner not feeling jealous of them. You should trust your partner not to act on crushes and to tell you if a crush is getting out of hand.
2b. A monogamous person has a crush on someone other than their partner. What typically happens next?
If the crush does not seem serious there is no reason to get worried. But if they feel themselves becoming increasingly attracted they need to distance themselves from their crush. If the crush persists they need to inform their partner and figure out a serious plan to avoid their crush as much as possible.
2c. A polyamorous person gets an STI. What typically happens next?
Hopefully they inform all their partners. I think this is typical but a sizable minority will not promptly inform everyone.
2d. A polyamorous person hates their partner’s other partner (their metamour). What typically happens next?
Hopefully they communicate this to their partner and find a solution. But a lot of the time this is just going to ruin the first relationship. If your partner is bringing around someone you hate then you are going to start feeling negatively toward your partner. Monogamous people are not immune to these problems. Your partner can have friends you hate. But in a committed monogamous relationship the relationship comes first. And it should come first by a lot.
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. What typically happens next?
If someone you are close to needs to go to the hospital, you take them. Cancelling a date is not a big deal. Mono vs poly isn’t relevant here.
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 have already talked about family instability. But there are other aspects of poly that would be magnified in a poly society. For example endless sexual competition. I imagine a society where everyone is constantly checking Tinder for their entire adult lives. Such a society will have much more drama and much less trust. If you introduce your friend to your girlfriend there is no telling whether he will start hitting on her. Can you even trust your brother?
A ton of women are going to feel extremely hurt when they get older. Men prefer younger women. This hilarious OKC graph comes to mind. Most men will date younger women if they can. Most women are going to feel betrayed when their husband of twenty years starts dating, or even trying to date, college students. Pretending that women are not insecure about aging is pure fantasy.
Younger men are going to have a brutal time finding partners. The most attractive young men will be free to have tons of partners. Even worse younger men will be forced to compete with older men who have much more resources. These effects are already causing quite a lot of men to complain. But things could get much worse.
This seems like a society where men feel abused when they are young and women feel abused when they are getting older.