My relationship with Adam, my husband and nesting partner, is not better or worse for being open per se. It's better because we actively work on having a romantic relationship alongside all the practicalities of living together. My relationship with myself is better for being open because I am not trying to forcefully jam myself into the ill-fitting monogamy box. I did try for a bit, but it negatively impacted my own wellbeing, which then meant I couldn't bring energy into my marital relationship.
Admittedly, I never experienced jealousy. I was never afraid of "losing" him largely because I never saw him as a possession. It's not because I love him less than someone who feels jealousy, I just have no fear of him leaving me "for someone else," or at all. (I'm terrified of him dying young, that would be the thing that would break my heart.)
Unpacking the implicit meanings in the words you currently associate with your existing relationship, making them explicit, can help with understanding what you value, and where you feel any overlap with another relationship would feel threatening, and where your current comfort zones are. (These can also change with time).