For me personally,
if a person isn't willing to allow partners to meet-there's too much risk that they are lying-and it's not worth the risk that they may also be lying about their safer sex practices.
Therefore, we have an agreement that before any new partners are added, they must meet all of us and vice versa.
THAT SAID-there is no requirement that we continue to socialize that way. Only that we meet, so we know that everyone is aware of the addition of a new sexual partner.
As for the details of "secondary" or "just sex play partners", it is arbitrary. We don't care if they are just fb, fwb, gf, bf, etc. If there are other sexually involved, then we need to meet. That meeting could be via phone/email whatever.
Ironically, this has never been an issue, because all of the poly people we've encountered were the same way-except one and she was new to poly-but decided after hearing about it that she felt hte same way too.
"Love As Thou Wilt"