There is not neccesarily a bridge between mono and poly. I don't neccesarily think that I would HAVE to live poly - I lived mono quite happily for years. Then I happend to really fall this this guy after had met my husband and then I discovered that I liked the idea of having two men. I know some people who have transitioned from having multiple partners to having just one, I don't really think that is neccesarily any different from going, let's say from dating just girls to dating boys (been there, too). Sometimes life pulls you in a different direction. That is not to say your life before was not meaningful. But then again, folks tend to regard whatever relationship(s) they are having at any given time as the best, and therefore downplaying the value of the past.
Cheating is of course not poly.
Poly comes in many shapes and sizes. I think polyfidelity comes closest to what I consider my thing