Both parties are interested in non-monogamy
This factor, if you are already in a relationship, is basic. Be careful: if your question is: “How can I convince my partner to have an open relationship?” you are going astray. It’s about both of you being genuinely interested. Otherwise it is not healthy. To this we must also add a caveat: there is a huge difference between people who absolutely do not want an open relationship and those who do but are afraid because they cannot visualize how things would work. This is completely common and does not mean that it is not a good idea. The key point is in communication. Which brings us to the next factor …
You and your partner have different sexual interests and fetishes.
It happens a lot. There are couples who have different sexual needs but want the same emotional connection. It may be that one has a higher libido than the other, or that, outright, falls into the asexual spectrum. It may be that one likes sadomasochism and the other doesn’t. In this case, it is worth considering opening the relationship so that both parties are sexually satisfied.
Are you willing to speak honestly
Open relationships are not agreements reached in a single conversation. Entering a non-monogamous relationship is a whole process in which the couple discovers and communicates what they are expecting from this stage and rules and dynamics are polished based on this. Can they sleep at other people’s houses? Are they going to tell about their adventures? Can they have unprotected sex? Can they have it inside the common house? Is polyamory an option? All those details have to be defined with complete honesty.
You are in an honest and trustworthy relationship
Being non-monogamous requires more trust and honesty than a normal relationship. If these factors are already firmly established, it is much easier for the open relationship to work. Otherwise, problems could arise in which there is no confidence that the members of the couple are adhering to the rules that they previously established. Think about it: if you can’t be honest, can you share the things that make you feel vulnerable and get the reassurance you need? Probably not.
They have different sexual orientations
Let’s say you’re in a heterosexual relationship and discover that you are actually bisexual and want to explore your same-sex attraction, but at the same time you love your partner deeply and don’t want to cut it off. In that case, one option is to open a relationship to being with people of the same sex but not the opposite.