My partner has a relationship status of "in an open relationship" and has listed me and my metamour as "Partners" in the family section. He did create a page for the both of us, so he could have "In an open relationship with..." but I was neither here nor there about that.
My metamour has "in an open relationship" and has listed him as Partner.
I (Mono) have "in a relationship" and have listed him as Partner.
The creation of the page on FB for the two of us seemed to get some mutual friends' knickers in a twist ("OMG, he's being arrogant and bragging, and..." <sigh>), but he wanted to give us both billing rather than give nobody billing, and it seemed like a decent way of doing that.