The key to any relationship is the factor of consent. In the context of relationships, consent is the mutual agreement upon behaving in certain ways --and in particular, sexual ways. For example, in a typical monogamous relationship, there is an agreement --explicit or implicit-- between both parties not to have sexual or romantically emotional relationships outside of their relationship. To break this agreement is a breaking of the consensual domain of the relationship, as you deny your partner’s humanity against their will, which was solidified in a mutual agreement. As, fundamentally, what consent is can be boiled down to the philosopher Immanuel Kant’s famous humanity principle, which is to treat people as an end in themselves, rather than as a means to an end. Which is to say, do not use people for your own ends, and especially do not do so without their knowledge. As in doing so, you deny the person’s humanity and treat them as lesser than yourself, which does great harm to them and is morally wrong as a result. We can see the same in the domain of sexual activity: if one partner clearly does not want to have sex, and the other does not take “no” for an answer and decided to coerce their partner into having sex, that is not consensual sex.
The same, of course, goes for open relationships. Everyone involved needs to understand everyone’s intentions transparently. And if one or more parties do not agree with such intentions, they must not be coerced into agreeing with them or acting in accordance with them. Many people, for instance, come into polyamory with the intention of having a sexual free-for-all. If that is what you want, you must make this clear to those you are pursuing. But, if those you are pursuing are uninterested in such a free-for-all, but rather, are looking for committed polyamorous relationships, it would be wrong and against the rules of consent to attempt to persuade them --really, coerce them-- otherwise.
When it comes to consent, the ordinary rules apply. When someone says no, or has negative body language towards certain actions, you are not to act in that way or coerce anyone into allowing you to act that way.
In the context of this discussion, we are talking about committed open-relationships.
There are rules to follow when approaching consent in polyamory that are a bit different. One is establishing that you are okay with your partner having other partners, and being honest when having new partners. It is on you, to be honest with your emotions here and not just go with the flow. If you deep down are not okay with your partner seeing other people, that is something you need to address. This is apart of informed agreement: if your partner sees others, but you did not tell them this makes you feel bad, you’re denying your partner the facts of reality and, in turn, are making them act in ways they otherwise might not have. And in doing so, you are denying them a legitimate consensual open-relationship.
If you want to have a new partner, you need to tell your existing partners. If you do not, they are not consenting to you seeing other people. This, fundamentally, might result in the dissolution of the relationship due to feelings of betrayal. Withholding such information is a way of abusing people --intentionally or not-- as, in order to consent, both parties need to be informed of what is going on. Let’s give an example: let’s say you are in an open relationship and your partner wants to see someone else; when you ask about them, you find out that this person might be abusive towards your current partner, so you disapprove of it and say that you’re not comfortable with it. Perhaps you’re worried that this potentially abusive person might harm your partner, or even try to isolate them from you and other loved ones. If your partner simply did not tell you and went to go be with this other person behind your back, that would be violating the consensual aspect of a committed open-relationship and, indeed, might very well result in an infringement upon it. This would be different if the agreement was “have relationships with whoever you want with no strings attached,” but that isn’t the case in a committed open relationship. A committed open relationship opens the door to other partners but doesn’t necessitate them. You have the leeway in an open relationship to not approve of your partner’s other partners. That is why communication is so crucial: without it, consent in polyamory is nearly impossible.
Fundamentally, open communication is crucial. If someone doesn’t approve of something, do not do it.