'And when an open relationship is stressed, the clear solution is to shut things down.'
You don't have to. And you probably shouldn't.
New secondary relationships play to the id. Thanks to the biochemistry of NRE , you're basically a stoner obsessed with a shiny toy. If someone snatches that toy away, you will not only resent the snatch-er, but the toy will take on unmerited importance.
Instead of stepping away from your secondary connections (and running the risk of resentment and obsession), consider changing relationship boundaries. Dial back to friendship. Continue being physical, but reserve certain acts for the primaries. Leave the boundaries where they are, but limit how often secondaries see one another. Etc, etc. Primaries can reshape their outside interactions, with the understanding that things can and probably should change again in the future.
The bottom line is: the more you treat nonmonogoamy in absolutes, the more you're going to trip yourself up. There are always more than two options (usually many more), and no choice is written in stone.
(PS. How smooth am I, embedding an MJ video? Huh? Huh???)