One needs to understand the sexual mores of the times. Once Cromwell's Commonwealth was replaced by the Restoration in 1660, the English began behaving sexually the way the French had
always behaved. These tendencies continued until the trial of Queen Caroline (wife of George IV) for adultery by the House of Lords, the death of the drunken lech William IV, and the ascension of Victoria in 1837. After 137 years, certain English sexual traditions ended: debauching the English serving wench, marriage in which the first five years were chaste (until the heir to the property was born) followed by serial infidelity, and rampant illegitimacy.
During Franklin's era, the favorite epithet was "bastard" because more often than not it was true. It was the Golden Age of Illegitimacy. Franklin was a man of his era who had pious intentions of changing his mode of behavior, but who was very much a part of his time.
Age may have made Ben chaste. Fortunately, they didn't have Viagra in those days.