If your partner having multiple partners while you don't is an unacceptable outcome for you then you definitely ought to get clear with your partner on what you'll do about it (collectively) if that happens before you start inviting other partners.

From what I've seen (monogamous man in a very poly-friendly social environment) yes, this is an issue that comes up, and yes, it's an expression of partners' insecurities. (This isn't intended as a dismissal; people have insecurities and relationships need to deal with those insecurities somehow.) It strikes me as similar to the issues some people have with partners who are significantly more romatically/sexually experienced than they are. In some ways it's also similar to the issues some people have with spouses who out-earn them financially.

One poly married couple I know deals with this by the wife, who is far more socially adept, helping her husband find other partners.

One approach you might consider is asking whether there's anything extra your primary partner could provide during the periods when she's more connected than you are that would make you feel less "raw end."

I infer from your assumptions that you're straight, but if not you might find it easier to find male poly-compatible partners than female ones. (In my experience it's less that women have an easier time finding dates, and more that it's easier to find male dates.)