Although there are many thoughts on this topic, one might argue that the tone of Shakespeare's "Sonnet One" (which is, to be more specific, the first of a series of seventeen sonnets themed around procreation) is cautionary.
If we look more closely, this poem has several movements to it, each of which serves a different purpose, but ultimately moves the argument closer to its prophetic finish:
In the first quatrain (a stanza consisting of four lines), Shakespeare addresses the subject of the poem (a "Fair Youth") with a general argument explaining the need for procreation in order to pass on the world's beauty.
In the second quatrain, Shakespeare admonishes the Youth for his unwillingness to procreate and to, thus, allow his own beauty to become shallow and a matter of self-interest.
In the the third quatrain, Shakespeare offers up compliments to the Youth and again relates his belief that the Youth is making a mistake in choosing not to procreate.
In the closing couplet (a stanza consisting of two lines), Shakespeare offers up the tonally potent warning to "[p]ity the world" and to recognize that the failure to procreate is parallel to allowing one's best qualities to be swallowed up by death.