For myself, yes, I very often cheated in past relationships. The cheating was always due to something lacking in a relationship and strong emotional ties to the person I eventually ended up cheating with, never a one night stand or a purely physical attraction. So I was always poly but never knew the term and never had a partner willing to discuss it, as I did try with a few prior to cheating.
My hubby and I started as an open relationship which turned more towards poly as we discovered we were in love yet still dating others whom we cared deeply for. Our others did not work out and we became mono for a long time. My "new" poly attitude came about not due to a lack so much as the fact I'm bisexual and, while N gives me all I need and want in a man and I don't feel I'd ever want another male partner, he cannot be a woman for me. And I had, over time, fallen for a female friend of ours who had also fallen for me.
So I can see where the thought that infidelity, either physically or emotionally, can be the doorway to poly. I'd hazard a guess and say it's pretty common. How would you know you're truly poly unless you developed feelings and desires for another while already in a relationship?