Mattie spends most of the book more or less deliberately oblivious, until the Fascist rival makes a mean joke about it. The closest they ever come to talking about it is when Mattie says the deliberately cruel thing during an argument -- it's something along the lines of "you're not responsible for me, you're not my spouse" -- which, not unreasonably, is what prompts the Flea immediately to pack her things and move out.