"All I know is I'm clean as a whistle" sounds like a denial of a rumor or slander that she has a
sexually transmissible / transmitted disease. "I never said nothing"--about a sexual liaison. Presumably someone told him something like, 'Why are you messing with that whore, you'll catch a disease from her.' (Cf. "Surrender" by Cheap Trick.)
"All I know is I'm clean as a whistle" sounds like a denial of a rumor or slander that she has a sexually transmissible / transmitted disease. "I never said nothing"--about a sexual liaison. Presumably someone told him something like, 'Why are you messing with that whore, you'll catch a disease from her.' (Cf. "Surrender" by Cheap Trick.)