Yeah, they used to have a partner option in the family section, but they later removed it. But there is one thing you can do: you can add a life event. From there choose relationships (or something similar) and choose started a relationship or started an open relationship with... and there you can add many people at once. I did that, so then it just popped up in people's newsfeed that 'Mya is in an open relationship with rory and Hank' (with links to their profiles). It's not the same thing because it won't be visible forever, but at least it shows up to more people because it's a life event instead of a regular status.
Me: bi female in my 30's
Living with Hank (partner) and Will (friend)
Metamour: Dahlia (Hank's partner)
Recent ex: rory (also dating Dahlia)