To clarify my remark that it can be difficult to tell how out someone is about being poly/open -- I wasn't really talking about walking past a stranger on the street and being able to tell whether they're poly.
Rather, I am talking about being able to discern how out someone who you are in a relationship with (or are becoming intimately involved with) really is. Especially if that person already has a primary partner. Because IME that's often not really obvious. People often don't think clearly and honestly, or communicate clearly or honestly, about how out they are or are willing to be.
Consequently, it's common for nonprimary partners to end up getting treated more or less like a secret -- regardless of whether they want or would agree to be closeted. That's a VERY common problem for nonprimary partners, especially for solo poly folk. Often this truth only emerges after a nonprimary relationship has become well established. Months or years down the line, the primary partner starts stumbling over previously undisclosed and non-negotiated (and non-negotiable) boundaries, exclusions, or requirements. This can be especially painful if your partner has a very public primary relationship, yet treats you like a secret, and expects you to be complicit in this secrecy.
Many people (especially straight primary/married poly couples) are quick to justify this closeting as a practical necessity driven by professional, legal, financial , or family concerns. Sometimes that is very clearly the case. (I mean, hell, it's still legal to be fired for being gay in over half of US states.)
However, I've also known several poly/open primary couples, even married ones (a few living in conservative states or communities), who do not conceal their additional relationships -- and the sky hasn't fallen. They haven't lost their jobs, their families haven't disowned them, their children haven't been confiscated, they haven't been evicted.
So, respectfully, I'd like to submit that perhaps such justifications often are really based more on assumptions that also conveniently happen to preserve couple privilege.
I say this because when I speak to poly couples who choose to stay in the closet and who expect their additional partners to keep this secret, I've found that usually they appear to be mostly clinging abstractly to those assumptions and fears. Generally they do little or nothing to research the facts about those risks, or to explore options to mitigate those risks which don't involve treating nonprimary partners as a secret or requiring them to closet or censor themselves on your behalf.
What do couples get out of preserving couple privilege? The many, many social recognitions and benefits that come with presenting as a committed primary couple, especially a hetero married couple. Plus the personal benefits of partners reinforcing primacy to each other. ("You're really #1, because I'll always put any other partners needs behind yours.") But if you have additional partners who stay closeted on your behalf, those benefits don't come free. Someone else is paying for them -- perhaps unwillingly.