She will definitely have to be a secondary, and beholden to what the primary says. Only she can decide if she can live with that. And, she is free to date other people, even if she chooses not to be.
As for Matt, I think he stays with you because he values all the good times you two have had in the past, and has faith in the future. His faith may have taken some dings, but it is still there.
Love means never having to say, "Put down that meat cleaver!"