Very interesting thread. My Mum (I believe) had a few affairs but stayed with my Dad, she moreorless admitted this to me once after my Dad had died saying that she was never very happy in their marriage but it wasn't the done thing to leave in the 1970s.
My exhusband had an affair, well, lead a double life to be correct, for 5 years, I didn't have a clue, he had a child that I had no idea about. He has been happily married to the woman he had the affair with for nearly 20 years now.
My best friend had numerous affairs which I berated her about until, after finding out about my ex-husband and, being single again, had an affair with a married man. My self-esteem was such that I grabbed the first sign of affection offered to me, yes, it was very wrong, but my headspace at the time couldn't see that. I ended the relationship, she found out about us, he offered to leave his wife, I declined and, to my knowledge they are still together.
My long term partner had an affair for a year before I found out. They are still together.
A friend of mine in her 70s has had countless affairs and is in an unhappy marriage but will not leave her husband because of her loss of financial security.
Another friend had an affair whilst she and her now husband were married. They have been happily married for many years now and are happier than they ever were in their previous marriages.
You cannot make a sweeping statement that 'leopards don't change their spots' as each circumstance of an affair is difference.
I am not stating that having an affair is right but, sadly, things happen in life and, if only it was as simple as people thinking, I am not happy with my current partner therefore I will leave (and deal with the fall out/children/financial mess) and start again. From my experience, a lot of people just stumble upon that other person that makes them happier.