Wow, this is a hot thread! Have not read quite all of it, but want to add my response to the OP. I was a poly virgin for a while. I was raised in a very strict religious environment: sex belongs to marriage only, and marriage is between one man and one woman only. I did not get married as long as I believed in that all. At some point I could not believe in the system anymore, and that was the time when I also started re-thinking the relationship issues as well as issues considering sex.
My ponderings lead me to the belief that honesty and openness in communication in relationships are of far more importance than sex. So before ever having sex with anyone, I had a poly mentality. Also, I never really had any romantic relationships before that either, so in that sense my history is very different from the OP's.
Anyway, I think there are many things positive in the fact that I started being sexually active very late and after a considerable amount of processing it in my mind. My first partner had to wait for me to be ready, and the opening came little by little. They did not look for sex anywhere else during this time, but that would have been ok for me - we did discuss it and they understood what I meant. It was a very beautiful process and I can happily look back to it now and feel good about my life then and my life now.
I am a woman with two male partners: CJ (legal husband) and Mark (no label added).