I am right on the imaginary line between mono and poly. I opened up initially at my husband's request, thinking that he would find a girlfriend and I would learn to have her as a part of our family. Then I found a lover. Then the marriage fell apart.
Now... I love my lover, and I love his primary and their child. I don't know whether or not I'll find someone who will fit with this family of mine or not. I suspect that I would give up being a secondary lover for the opportunity of a long-term committed partner, if I had to. This is hard to say. I didn't expect to fall for the lover, much less the whole family dynamic, and there are so many benefits to having an OSO I never ever suspected.
I also find having more than one partner draining. When I was still with my husband, there was a lot of drama, but I think maybe it had to do with other issues. I've tried dating since. Maybe it's draining because they were a poor fit?
There was a shitload of pain involved with my growth and change process. I'm glad I decided to try it. I'd be open to other poly relationships, but very cautious about with whom and how they function. I'm not convinced that it is better or worse than a mono relationship.