Is there ever a suitable reason for infidelity in a relationship?

I think there are times when infidelity is understandable, even though I would not call it justifiable. For example, a trusted friend once confessed to me that she had had an affair. I understood the frustration and loneliness she felt in her marriage at the time. I listened in what I hoped was a compassionate manner, while acknowledging that is was wrong. She also believed it was wrong (and she had cut it off a long time ago). But I urged her not to punish herself over it.

So no, I do not thing there can be a suitable reason, but it does not follow from that that I would automatically condemn someone who has an affair. I myself have been guilty of "emotional infidelity", and I am trying to come to terms with that.