act of sin. Furthermore, the depiction of gluttons in that time also suggests a disgusting, foul, and dirty appearance and manner. So, the speaker is also expressing his disgust with the young man's actions.
In the final line of the sonnet, the speaker tells the young man that to pass on his beauty to his children is a debt that he owes to the world. The world (or nature or God) gave him beauty, and so he should not be selfish with it. Once again, the speaker emphasizes the idea of time with the word grave. The young man will die eventually. And if he dies without having a child, he will die the death of a "glutton" -- the death of a selfish and sinful and foul creature.
Throughout this poem the poet uses death imagery: decease (line 3), buriest (line 11), and grave (line 14). The speaker is an older man (as will be revealed in later sonnets), and he has far more experience to understand how quickly times passes and how soon death comes for all of us. In fact, in some of the later sonnets (especially 71, 73, and 74), the speaker will directly comment on his own approaching death and the effect it may have upon the young man. In this poem, then, three themes are united: beauty, love, and death. The speaker's love for the young man urges him to convince the young man to do what is best for himself and for others because time and death will come far sooner than he expects it.