I'm sorry your thread got hijacked OP. It is an interesting question.
In my circle of friends I (36F) would honestly be surprised if I discovered any of the married couples I know were having affairs.
There has been one divorce amongst our friends, when the wife cheated on her husband with her personal trainer. It was an exit affair, she was discovered/confessed (I'm still not sure) after a couple of months and both agreed to a quicky divorce as they admitted the marriage had been on the rocks for a while longer than that. No kids.
She was so embarrassed about the affair she begged her husband not to tell people, has never spoken to me again as she knows I know, and is still with the AP and tells people she met him on Tinder.
I do think 'birds of a feather' is true though, as in my youth I had an affair and became the mistress of an older man, who wasn't married but had a long term GF he married months after our affair ended. He was a bit of a serial philanderer, and I met him through a friend of mine who cheated on every man she had ever been with and eventually had an affair with my boyfriend. Our friends all had exes they slept with on occasion despite having other boyfriends/girlfriends.
We were young though, and some of us grew up and became clearer on our own values and perhaps grew in our self-esteem.
My AP and his wife are, to the best of my knowledge, still married and my ex-friend and ex-boyfriend are also now married. I would be surprised if the two serial philanderers had kept it in their pants all these years though...
There is a certain type who cheats repeatedly and I am not interested in friendships with those people, so I think my friendship group is fairly self-selecting.