I was in a monogamous relationship for seven years, and I'll admit, was quite happy and content. I did not feel this overwhelming urge to date more women, nor did I feel trapped in my marriage. Granted my wife is not the jealous sort. If she ever caught me staring at or admiring other women, her response was more along the lines of "she's cute" or "yeah, I'd do her." We had talked about an open marriage early on in our relationship, and while it had its appeal, that's all we ever did. Talk.
Have I always known I was polyamorous? I guess the answer to that is no.
However, I have never wanted to be single, that much I do know! I longed to have someone to love and share my life with. Whether that is one person or three, isn't really the point. Adding extra lovers doesn't make make me any happier than I already am, but they do have positive impact on my life.