Solopoly has a list up of do’s and don’t’s when it comes to treatment of a non-primary partner. The list:
Honor time commitments and dates.
Listen to and honor your non-primary partner’s concerns, needs, and feelings.
Make your non-primary relationship a priority.
Offer reassurance and understanding.
Embrace your non-primary partner’s world.
Keep your promises.
Support good metamour relations.
Invite non-primary partners into negotiations and decisions that affect them.
Clarify your boundaries and commitments BEFORE you begin a new relationship.
Fully disclose your constraints, agreements and boundaries.
Speak up about fairness toward non-primary partners.
Assume good intentions.
Don’t violate agreements.
Don’t conflate “fairness” with “equality.”
Don’t bail at the first bump.
Don’t default to playing the go-between.
Don’t foster competition or conflict among your partners.
Don’t pretend the dynamic of your existing relationship(s) will not change.
Don’t make it more complicated than it needs to be.
These are mostly good suggestions. Most of them apply to any relationship, not just a non-primary relationship, and they are generally rather intuitive. However, I’d like to add something to the list that’s somewhat counterintuitive:
Do Take Sides
This does not necessarily apply to only non-primary relationships, but it applies to any situation in which you are dating multiple people who have exposure to one another. In such a situation, conflict is inevitable. If you date for long enough, your partners are going to have a conflict with each other. In such a situation, your instinct is going to be to remain neutral , to facilitate discussion, but not to exercise judgment. In my experience, this is a mistake.
When your partners have a conflict, you’re going to have an opinion. I’d be willing to bet you’re going to have a pretty clear opinion about who did what wrong, who should apologize, and what they should apologize for. Hiding that opinion doesn’t help anyone. Firstly, it harms the trust you’ve built with both partners. Hiding things always does, and hiding something so relevant and important harms the trust to a greater degree.
Secondly, when you’re in a relationship with two people having a conflict, you have a lot of power to influence how the conflict goes. And as Spider-Man has taught us, that comes with a corresponding responsibility. By being in a position of trust with both people, you are possibly the only person who can talk straight to both parties and have them actually listen. When one of your partners is behaving unreasonably, you are one of the only people who has the ability to talk them down. If you abdicate that responsibility, your partners have to solve the issue themselves. Maybe they will, but they’d stand a much better chance with an effective mediator.
The much better alternative is to pick a side in the dispute. If you think one party is right and the other wrong, say so. If you think both are wrong, say that. If you think one is very wrong and the other is only a little bit wrong, say that too. In almost any dispute, both sides have made errors. Point them out,. But do not try to remain neutral. It’s easy to fall into the “both sides are wrong” trap. It’s easy to point out minor infractions on both sides are pretend they are equivalent. Don’t do it. One side is almost always more wrong than the other. Say which one.
The other thing to remember, and I can’t stress this enough, is do not always choose your primary partner’s side. The whole thing only works if you give your honest opinion and overcome your biases. Your primary being your “top priority” does not mean that you always take your primary’s side in a dispute. Sometimes, your primary is going to be in the wrong. It happens. It’s up to you to say so.