Which, like a canker in the fragrant rose,
Doth spot the beauty of thy budding name!
O! in what sweets dost thou thy sins enclose.
That tongue that tells the story of thy days,
Making lascivious comments on thy sport,
Cannot dispraise, but in a kind of praise;
Naming thy name blesses an ill report.
O! what a mansion have those vices got
Which for their habitation chose out thee,
Where beauty's veil doth cover every blot
And all things turns to fair that eyes can see!
Take heed, dear heart, of this large privilege;
The hardest knife ill-used doth lose his edge.
The final  metaphor  is the knife, which used clumsily will be damaged or “lose his edge”. The phallic implication is unmistakable. So, Shakespeare ends on a note of humour.
Sonnet 95