There is no doubt that a couple must have a strong and healthy relationship before they think of opening up, they must be prepared for the changes a Poly life will bring to their relationship (well, as prepared as one can be)and their family and even when you expect it, still allow for it being even harder than that! Because long term monogamy allows you to get to know a person so well, to have habits and some unspoken communications and if you happen to have very good open communication and a no secrets policy with each other also, well that makes for a pretty good base to start with.
To an outsider just meeting you, this strength is encouraging, it communicates that this isn’t a desire borne out of lack of satisfaction with each other, seeing how well a couple communicate with each other proves that they have learnt the skills for good communication, for the person interested but not emotionally invested, this is a good thing.
Fast forward some time and this strength seems less a strong admirable thing, but that outsider now wants to be inside this circle of strength and it feels a bit more like an impenetrable fortress. When does this happen? At what point do these issues start to arise? Well who knows? It, like everything varies between people but it is an important thing to be aware of. A new partner might at one point feel proud and secure when s/he tells antagonists that ‘No, they have a wonderful relationship, nothing was wrong with them et al’ but months later, when emotions are invested and these people are all living together, the habits a couple have together can mean the newbie can be an after thought or forgotten accidentally. The unspoken communication can feel like a secret language that you are not and might never have, the no secrets policy between them as well as shared history and shared confidences can feel like a depressing obstruction to getting to know them as well as they know each other
There is no way to prevent this from happening but being aware that this can occur means you can open a dialogue, recognise where the pain comes from and find inclusion strategies to help.