It is obvious that a known affair is irrelevant. Look at Clinton. Getting a little gratification from a bimbo under the desk is one thing but if the entire world knows the only security involved is the physical damage that might have been inflicted by the suckee's wife.
A KNOWN affair? Prior to yesterday, almost nobody knew; this was a complete surprise to almost everyone. Also, the woman wasn't at all "a bimbo" - she was his biographer who herself was married. She tried to end the affair but he relentlessly continued to pursue it through e-mails. What awful judgment on his part since he had to know that there is technology to monitor the e-mails of high-ranking government employees. It was those e-mails plus a tip that led to his being discovered.