In my life in general, I've found that being vocal about everything I am interested in has rewarded me.
I'm openly poly in my life. I'm a political activist and don't mind talking about my philosophy. Both of those things are important to me so I mention them to friends, family or anybody else in my life who cares about me enough to have the conversation.
There are "closet cases" everywhere. I've had people that I totally didn't expect respond to my discussion of poly. I have a knack for finding people interested in poly (and in BDSM for the same reason) or open relationships.
It's very likely that people around you are or are considering an open relationship that are too uncomfortable broaching the topic. To all of those people YOU look like the "dedicated mono couple" that you're ruling out because you don't know.