Rules Are Made for Those That Break Them
There is a school of thought in the polyamory community that you shouldn't have rules, only personal boundaries, which you make agreements around. This may seem like a subtle distinction, or maybe even just wordplay, but it's an important difference.
In poly, making a distinction between rules and boundaries is important, because it's no longer about two people. Two people in a relationship with only each other can make rules with each other all day long as long as they both agree to them. In poly, when you create a rule between two people instead of stating your personal boundaries, the rule also affects any future third party but they didn't have any say in it.*
Rules are declarations, whether agreed to or not, that you will enforce someone else's behavior otherwise they will face consequences. Boundaries are clear definitions that you state about what you are comfortable with, and you act on your boundaries, instead of demanding that someone follow your rules. When it comes down to it, you are deciding whether you're controlling someone else's autonomy, rather than your own.
Polyamory can be scary without rules. You can feel as though it makes it easier for your partner to sneak around or do something that you are uncomfortable with, and you may think that if you put rules in place, you can stop your partner from doing things that make you feel bad. The problem is, rules are a false sense of security. You can't control someone else's behavior by making demands.
Look at it this way: Do you trust your partner? Do you trust them to respect you? If you do, then why do you need to make demands? If you make your boundaries clear, then you can trust your partner to respect your boundaries. If you don't trust your partner to respect your boundaries, then why are you with them?
Another thing that makes boundaries important is that you no longer have to worry about someone following only the exact rule rather than what your true concern was.
Everything that is usually demanded in a rule can be restated as a boundary. Here are some examples of when a rule is restructured into a healthy boundary.
"You are not allowed to have unprotected sex with anyone but me."
The Problem: This is one of the most common rules in polyamory. I will never argue that safe sex is a bad thing - it is extremely important. But the issue here is that people make mistakes, and structuring this as a demand can mean that if your partner does have unprotected sex, they might not tell you for fear of the consequences.
"I will not have unprotected sex with anyone that has unprotected sex with other people."
The Solution: This boundary, restated, makes it clear what your actual need is. You require physical safety and you make it clear that that is what is important. You should also explicitly state that if your partner has unprotected sex with someone else, you would use protection with them until the results of a new STI test are cleared. This way it is less scary to deal with a mistake that comes up.
"VETO! You cannot date him or her!"
The Problem: This is probably the second most common rule, and the one that causes the most resentment if it's used. You are exerting control over two other people's relationship, and that's rarely something that works. Ever see the "forbidden love" movies, or teenagers that run away to get married because the parents say no? Yeah, it kinda works like that.
"I cannot be in a relationship with someone that is toxic to me, and that includes metamours."
The Solution: This boundary is a very firm one and is a reminder that you should distance yourself from toxic situations or interactions. You can and should always voice concerns with your partners over red flags you see in your metamours, but you can't make decisions for other people. Generally if you have a healthy, communicative relationship, your partner will eventually see the red flags themselves, and probably end the toxic relationship on their own. If they do not, then protect yourself and leave them. It's easier said than done, but it's less painful in the long run.
"You can't spend more time with them than with me."
The Problem: I already covered this rule in another article, but it bears repeating.
"In order to feel cared for, I need to spend time with you."
The Solution: The wording of this would change based on how much time is needed, and is something that can change constantly (and needs to be communicated when it changes). That said, it is important not to compare your relationship to your metamour's relationship, at least because you are different people.
In the end, these are all examples of restructuring boundaries made to protect you instead of rules made to restrict others. They end up being healthier for each relationship in the long run.
* This paragraph was modified from a comment on Reddit - credit where credit's due!