I wouldn’t say that an affair reveals who a person is at their core, no.
It reveals a part of who that person is at that point in their life. And likely the weakest part of them, at that.
Consider that an affair is a part of a person they want to hide. Not their best self, not something they wish to shout to the world, "Hey, look at this part of me, this is the best of me!" Certainly the opposite is true. This is the part of the self that is clandestine, shameful, hidden, the part they don’t even want to admit to their closest confidante.
If we are to believe that an affair is "who someone is", then by extrapolation, would our reaction then be "who we are"? No. Because the premise is false.
Humans are complex. We have all done things that hurt other people, made bad choices, said things in anger. Were I to be defined by any of those things that I have done, my life would be much different now.
If we are to believe that because a person has had an affair that is the core of "who they are", this seems to allow that person no hope.
There is no redemption for anyone, then. If you have done anything wrong, then that is the "core" of who you are? Is there no redemption to be had?
I don’t believe that. I believe every person has the capacity for change.
I do not believe human beings are static in their existence. I look back over my life, and know that who I was at 6, 16, 26, 36, 46….and now….are all very different. Changes in my thoughts, my ability to love, my understanding of others, my compassion, all of my "core" has changed.
Things change daily. And I have to allow space for every other person to change, too. That’s where compassion happens.