«That thou art blamed shall not be thy defect,
For slander’s mark was ever yet the fair».
The poet is unable to maintain his disapproval of the young man, but he forgives without forgetting. The youth can blame only himself for the slanderous rumors about him.
That thou art blamed shall not be thy defect,
The poet notes that the slander pays an oblique and unintended tribute to the youth’s innocence, charm, and beauty: “For canker vice the sweetest buds doth love, / And thou present’st a pure unstained prime.”
The youth’s real problem, according to the poet, is that his morally ambiguous nature leaves him vulnerable to slander; his virtuous beauty masks a potential for vicious habits: “If some suspect of ill masked not thy show, / Then thou alone kingdoms of hearts shouldst owe.” The poet calculatedly appeals to the youth’s vanity in the hopes of encouraging upright behavior.
English audio from YouTube Channel Socratica
Summary from Cliffsnotes.com