I've basically always known, even though I didn't have terminology or context for it.
When I was young, the concept of "one person for ever and always" sounded silly and naive, I knew I could never hack that.
Oddly enough, none of my relationships ever felt insufficient... I never felt that being with only one person at that time was holding me back or not meeting my needs. I was usually so caught up in NRE that I didn't think of anyone else. And since none of them were LTR material, the wearing-off of the NRE spelled the end of the relationship before I got to a stage where I was interested in other people.
With my marriage, it started out the same way: we were enough for one another, and even though I was vocally poly when we met, it was my idea to close off the relationship while we built a solid foundation. Once we had that, my poly feelings grew, and we opened our relationship back up.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker