Unthrifty lovelinesse why dost thou spend,
Upon thy selfe thy beauties legacy?
Natures bequest gives nothing but doth lend,
And being franck she lends to those are free:
Then beautious nigard why doost thou abuse,
The bountious largesse given thee to give?
Profitles userer why doost thou use
So great a summe of summes yet can'st not live?
For having traffike with thy selfe alone,
Thou of thy selfe thy sweet selfe dost deceave,
Then how when nature calls thee to be gone,
What acceptable Audit can'st thou leave?
Thy unus'd beauty must be tomb'd with thee,
Which used lives th'executor to be.
In the first quatrain, the poet asks the fair young man (unthrifty loveliness) why he ejaculates (spends) his semen (beauties legacy) on himself, but he does it in a very graceful way, which, if one chooses to ignore the literal meaning, still makes sense. He points out that nature does not give, but merely lends, and since nature is generous (franck) she lends to those who are themselves generous (free).
In the second quatrain, the poet chastises the fair young man because he does not put to use the great gifts (bountious largesse) that have been given him by nature to spread around. He secondly chastises him as a userer who makes no profit out of his usury, but rather hoards everything (summe of summes) to himself, and does not produce offspring (can'st not live).
In the third quatrain, the poet points out that it is only himself that he robs (deceave) by his behaviour, and asks what acceptable account (Audit) he will be able to give of himself when death calls him away.
The final couplet re-iterates the idea that beauty not used to produce offspring will be buried with the fair young man, whereas, used, his beauty reproduced in his offspring will live to perform what is necessary for him after death.