Note to Twelfth Night, 1.5.241-242, "If you will lead these graces to the grave / And leave the world no copy"
Cesario/Viola is saying that Olivia is cruel if she lets her beauty, her "graces," die without having a child, a "copy" of her beauty. This idea, that a beautiful person has a duty to pass on his/her beauty by having children, dominates Shakespeare's first seventeen sonnets. Here are a few examples:
From Sonnet #1:
From fairest creatures we desire increase,
That beauty's rose might never die, . . .
From Sonnet #3:
Die single, and thine image dies with thee, . . .
From Sonnet #10:
Make thee another self, for love of me,
That beauty still may live in thine or thee.