Racy, sexy, unfaithful America? Think again. The majority of American married couples are faithful to each other, says a source that has long been an oracle of lusty lifestyles. None other than Playboy magazine reports that 75 percent of men and 82 percent of women said they had never had an extramarital affair, according to a "Fidelity Survey" published by the magazine yesterday. ......(excerpt)...... Playboy asked of the 1,432 adult respondents. Playboy founder Hugh Hefner declined to comment on the survey. ......(excerpt)...... Respondents overall estimated that 44 percent of men and 38 percent of women cheated on their spouse. .....(excerpt).........