Partner guarding seems like a lot of pointless energy expended based on insecurity; one of my aims in relationship(s) is to constantly improve to remain the highest-value around without resorting to possessive capture. Plus, it's better to let people move freely and self-select voluntarily, rather than give them ultimatums or threatening "if you ever" speeches.
I look at importance of relationships (love/friendship) based on the amount and sustainment of attention, mutual care, support, and trust. Sex is just sex to me without deeper affinity and common interests; a shared, social activity that is delightful without being black-and-white all-meaningful. It's also important to be sure other participants are truly on this same wavelength so that they don't end-up regretful or are otherwise left worse than before.
In conclusion: open works best for me, with a consideration for Mff or Mfff poly where f are pan/bi. I think Mfff tetrad of ever-reconfiguring pairs is more stable because there's no third-wheel issue. And all disagreements must be followed by bedroom activities, and stalemate disputes are resolved by last one who gets there wins.
# Keeper
1. You trust them with your life
2. Better together than apart
3. Good friends
4. Common goals
5. Complementary personalities