As a rule I think we Americans tend to separate a person's ability to govern well from his 'peccadillos', feeling that marital infidelity is a 'personal matter' between the Pres and the First Lady and that it does not affect his ability to run the country. I'm not saying this is right or wrong, just that it 'seems that way
Unless you're Hillary Clinton of course and the fac you were loyal and decided to work at your marriage, actually makes you even worse 