One of the big problems problem with explaining poly to people who are unfamiliar with the idea is that they often automatically picture their partner’s partners as adversaries.
This is a mistake.
If I’m dating two people, May and June, and I’m having issues in the relationship with May, I would expect to be able to talk to June about it. I would expect her to be supportive, and I would expect her to, generally, try to help me figure out the best way to deal with the issues with May. My partners don’t have to be friends, necessarily, but they do have to be well-intentioned. My partners have to want me to be happy. Part of that means wanting my other relationships to be happy. In general, I expect my partners to help rather than hinder the other relationships in my life, be they romantic or sexual or platonic or whatever.
If one of my friends is having issues with another one of their friends, and they ask me for advice, I try to help. I don’t try to obliterate their other friendships for personal gain. I see no reason to assume that relationships should play out any differently. I do think it’s important to tell my friends if I think one of their friends is a shit friend. But only in the case of shit friends. If I had a partner whose other partner was a shit partner, I’d tell them. The thing is, I’d be doing it because of the SHIT part, not the PARTNER part.
A good friend helps you out when you have issues with other friends. A good friend helps you when you have issues with a romantic partner. A good romantic partner helps you when you’re having issues with a friend. And there is no reason why a good romantic partner would not also help you when you’re having issues with another romantic partner.
I think if people could stop seeing partners as necessarily adversarial, and start seeing them as allies, a lot of other problems would fall away.