One of my OSOs married right after college and has been with his wife for over two decades. He had no idea that polyamory existed as a concept, and indeed, I don't think the term was even coined until after he married.
A few months after I met him he shocked me by admitting that he had had thoughts of suicide in the distant past because he had felt so stifled and unhappy in the monogamous life he was leading. Now, six years after he and his wife first opened their marriage to others, he's probably the happiest person I know. It hasn't been easy for him, his wife has struggled with his poly-ness and he hasn't had much luck finding lasting love relationships outside of me, but being poly is a huge part of his identity and I know he could never be any other way.
The majority of poly married couples I know started out as monogamous. I myself was never aware that monogamy was a conscious choice, instead of a given, until I was in my 40s.