[S]he hoped he would always think he had been as mad as hell, and not . . . not the way his face said he felt.
Infidelity (whatever that means to the couple in question) is such a big issue in a marriage that it cannot be simply swept under the rug.
But why is it such a big deal? Relationships sometimes peter out. Friends drift in and out of closeness, sometimes trading best friends. Why not spouses, too? Sure, judeochristian ethics frown on infidelity, but why? (I mean besides the obvious husband : wife :: Christ : church analogy. I've always thought of that analogy as arising precisely because of the ban on infidelity.) I don't have the answer, but I do have a thought: dignity.
William Peter Blatty's The Exorcist is both beautiful and insightful. Toward the end, Father Merrin (the old priest) and Father Karras (the young priest) are talking about what the demon seems to want from Regan (the little girl):
I think the demon's target is not the possessed; it is us . . . the observers . . . every person in this house. And I think---I think the point is to make us despair; to reject our own humanity, Damien: to see ourselves as ultimately bestial; as ultimately vile and putrescent; without dignity; ugly; unworthy.
The demon is trying to debase Regan and the others and take away their dignity. When I talk about dignity, I mean something deeper than pride and more lasting than the absence of embarrassment. I mean a recognition of your intrinsic self-worth, a recognition that, regardless of what seems to be or what people think, you deserve to have that worth recognized.
And dignity is what infidelity attacks. We all deserve, intrinsically, somebody on our side, even when the world hates us. It's why we all have a Sixth Amendment right to counsel and a right to zealous, loyal representation. And maybe that's why infidelity is so destructive.