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, dominants 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.