Extramarital affairs probably reflect more on the  neurochemistry of love and chance circumstances than on someone's character.  People who fall in love/lust lose a lot of their ability to make "good" decisions (being faithful) because instinct is overriding social conventions.  Evolutionarily speaking, and instinctually, it is "good" for a male to have as many children as possible with as many females as possible.  I bet a lot of those with "good character" (faithful) simply haven't fallen into the circumstances that would cause them to compromise their character.  Being a politician or a celebrity tend to give you many more opportunities to do so. 

Humans are not naturally evolved to be purely monogamous.  By insisting on monogamy, we go against our true nature.

"Vasopressin is the monogamy chemical. Only about three percent of mammals are monogamous; mating and bonding with one partner for life. Unfortunately, humans are not one of these naturally monogamous animals. The prairie vole is however and it is this furry friend which is responsible for our knowledge about vasopressin." 

I like the European way better, but doubt there will be change anytime soon.  I think Americans delight in seeing politicians fall from grace.