The etymology is instructive. Betrayal is not only a violation; it is an act with a direction. Something goes somewhere it was not supposed to go. A confidence shared in the dark ends up in the light. An intimacy offered in good faith gets handed to another party — whether that party is another person, or an addiction, or the betrayer's own needs for excitement, for escape, for a self less burdened by the weight of commitment.
What makes betrayal in intimate relationship so specifically devastating is not only the pain it causes but the retroactive revision it requires. When a significant betrayal is discovered, the past must be reread. Every remembered moment now carries the shadow of what was also happening, unseen. The holiday they took together three years ago now has subtitles. The evenings she thought he was working late now require re-examining. This retrospective revisionism is one of the cruelest dimensions of betrayal: it does not only damage the future; it contaminates the past.
Contemporary culture tends to locate betrayal exclusively in the erotic. But couples who stay in the aftermath of non-sexual betrayal often discover that what was violated was something more foundational: the implicit agreement that you will bring your inner life here. That this, whatever its imperfections, is where your most unspeakable things are supposed to land. When that agreement is broken — when someone chooses to be known more fully elsewhere — the injury is not to the ego so much as to the shared project itself. The thing you were building together turns out to have been built on different foundations.