There's nothing to say that's likely to happen. Yes, there are largish poly tangles that require road maps to navigate. That's not usually the case.
Who says your secondary won't regard you as a primary? Amongst our membership, Mono is an additional partner for Redpepper and Mono is monogamous--Redpepper is his only partner. My previous paramour was involved only with me, making for much the same situation (I'm married to CurlySquirrel).
Some tangles practice what they call polyfidelity, meaning that they only have sex with other folks in the tangle and it takes a group consensus to let anybody new into the group. Other poly folk require safe sex practices with partners who aren't fluid-bonded (which means everybody bonded has been tested for STIs), allowing unprotected sex only with bonded partners.
There are some folks who take additional partners because their primary relationship is sexless; their only sexual activity is with the secondary. This appears to be fairly rare, though I expect it to happen more often as poly garners greater acceptance.
And so on.
The most important thing about all of these situations are that they are negotiated by the people involved. If you're only comfortable being involved with a limited sexual network, then you find other people with the same proclivities and negotiate agreements that limit sexual exposure.
There's no single way to do poly.
When speaking of various forms of non-monogamy...it ain't poly if you're just fucking around.
While polyamory, open relationships, and swinging are all distinctly different approaches to non-monogamy, they are not mutually exlusive. Folks can, and some do, engage in more than one of them at a time--and it's all good.