Twice with women who were married but I was single. Both were in very bad marriages and wanted out. Both had suffered mental abuse and neglect and one of them regularily played away with different guys while the other started out being unfaithful with me and then I heard she had gone into a lesbian relationship after divorcing her husband.
I have, more than once. There have been times it was purely physical and other where it led to something more emotional.
Yea, I would in a heartbeat. But.... probably not if I knew the husband personally, and would like to think I'd break it off if it began to develop into more than a physical thing.
Having been with several married women years ago. I often wonder if any ever confessed to their husband?