This is true, but consider the situation for a moment.
You may be a perfect, upstanding, pilar of the community who never sets a foot wrong. But anyone who has ever done something bad will ultimately find it hard to confess. Especially if the repercussions are, or could be, severe.
You could be feeling incredibly guilty, but if your fear of the repercussions is greater than your guilt, then you will probably stay quiet until someone confronts you about it with evidence.
I know from personal experience of screwing things up that fear of the repercussions will make you hide it, even if hiding it makes the repercussions worse. You are just not thinking that clearly about the situation, and just focussing on the *now*. I believe that is what has happened here.
It doesn't excuse her actions lying to me before the affair, or events themselves, or the talking / meeting up after. But it does explain why she kept quiet.
Im not a religious man, but I believe there is a quote of "judge others as you would be judged". Implying that you shouldnt use a different scale of values to judge people if you cant live to the same values. My point being that I would have almost certainly kept a similar event secret for the reasons listed above, so I dont believe in holding that particular point against her.