Every time I stumble over some topic dealing with 'inclusion' I get reminded of one of my basic educational science seminars on migration. I think some of its contents could be applied to the issue at hand as well.
We were told that one of the basic faults one could make is confuse integration with inclusion. Especially in Germany there are many projects that have labelled themselves as 'integrative'. But that's the unhealthy way of doing it. You try to 'shape' the new element in the equation just like all the others are. If the core group consists of triangles, and a square wants to join them, it has to become a triangle for this system to work. Quite different from inclusion. If you want to include someone, you just let the square be a square and both sides have to adjust to the new situation.
Most of the people seeking a triad do just this. Especially if they are an established couple. They want the new person to fit both their tastes, want them to be this or that way and end up integrating someone into their two person circle. The original shape of the person gets lost.
Doing inclusion correctly means not forgetting about all the different aspects that are there in each person that has to be considered. It means that each piece of the equation has to remember that there are others as well.
The pitfall of wanting this or that relationship style/structure is having an already established blueprint in mind, how this should work out, how things should be from the own perspective without thinking about how the others contributing to the relationship(s) would picture that. That's an integrative behaviour, while an inclusive one would be a more relaxed approach considering the individuality of each person.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.