Let them tell you how they define their relationships.
And if they identify multiple people as their partners, don’t try to read into who is more important than whom, imagining hierarchies even if you’re told there are none.
I don’t see anything wrong with strictly sexual non-monogamy so long as it’s genuinely fulfilling and consensual for all involved, including the outside element.
But for those of us living in families, it can be incredibly frustrating when people use those concepts of open marriage to make assumptions about the structure of our relationships.
Many share homes in configurations like ours, or as committed triads or quads or complex networks of five or more.