McCain was an officer in the USN in at the time he started screwing around with Cindy, while still living with his wife & kids. 

In the services, adultery is a court martialable offense.  McCain was USN liason to the US Congress, a high profile position where he represented the Navy. His behavior was not to his credit and showed seriously poor judgement.

After plunging into this affair (and others), rather than trying to patch things up like Edwards is, he dumped his wife.  She had been seriuosly ill for years, raising his kids while he was a POW.  He started a new life by marrying  Cindy, a rich young thing. A few years later, he used her money to run for the Senate.  So it's no stretch to say his adultery paid for his senate seat.

Under these circumstances how McCain's philandering is any more or less a disqualification, in quality or quantity, for high office than Edward's, I do not see. Edwards lied to the press about his affair.  McCain didn't have to, he only lied to his wife.