«Those petty wrongs that liberty commits,
When I am sometime absent from thy heart».
In order to forgive the youth for his actions, the poet places himself in both the youth’s position and that of the mistress. In the sonnet’s first four lines, the poet mildly accuses the young man of committing small sins, but he then goes on to accept the youth’s actions given his age and beauty.
Those petty wrongs that liberty commits,
The youth’s behavior, so the poet seems to say, is natural and expected. However, what is even more expected is that others attempt to gain the young man’s affections: “Beauteous thou art, therefore to be assailed.” This reasoning prompts the poet to blame those who tempt the youth rather than the youth himself.
Forgiveness of the young man is mixed with reprimand, for he breaks “a twofold truth” — the poet and the mistress’ affair — when he begins loving the woman. Although the poet admonishes the youth, his tone is reserved, in part because he suggests that the youth and the youth’s beauty are two separate things: “And chide thy beauty and thy straying youth, / Who lead thee in their riot even there / Where thou art forced to break a twofold truth.” The young man is not at fault for coming between the poet and the mistress; rather, his beauty and youth “forced” him to act as he did.
English audio from YouTube Channel Socratica
Summary from Cliffsnotes.com