So is Mattie his first extramarital affair?
I'm wondering this because Francis as described in Dobbs' original novel and Francis as depicted in the teleplay do have some differences in this respect.
In the novels we're given some indication that Francis has played around on his wife in the past, though nothing particularly detailed about this is described in House of Cards (in the original version, before Dobbs retrofitted his and Mattie's affair into the re-write). In To Play the King, his rather tomcat ways are made much more explicit, including the reader being told that he's indulged in at least a couple of illicit affairs with female students he'd been tutoring. And of course there's his highly sexual romp with Sally Quine.
In the HoC teleplay, however, there's some hint that Francis is a bit naive in that department, as when Patrick Wolton makes an off-color joke about brothels, which it seems fairly clear Francis doesn't quite get although, to seem one of the boys, he laughs along and then later repeats it when upbraiding an underling who's been careless in his dalliances. The idea that he's been a longtime philanderer isn't given much play in the screen version and it's almost possible to suspect that Mattie might be his first foray into actual adultery.
Because this act is so pivotal and central to the story, it does raise the question of just how unfaithful Francis has been to Elizabeth in the past, or if in fact, this is a new avenue that he's decided to pursue as in keeping with his all-stops-out drive to win.
50 Is The New Cutoff Age.