See, with my Lucius, Narcissa tolerates the screwing around, because she knows it's no threat to her. She remains lady wife, unchallenged, and has the sort of sex she likes with him; he has play-toys on the side, mostly male, and exercises the kinks she doesn't care for with them. He's discreet and doesn't cause her public humiliation over it, and she's well satisfied.

It's not as if she's worried about losing his affection, after all.