Behind all of the talk of beauty, time, and kids in Sonnet 2 is the idea that we all get old and die. When we are young, we don't think much about it, but finally, it gets us all. The move toward the grave is the driving force of this poem. It's the opposite of beauty, and it wins every time. This is getting a little grim, isn't it? That's the point, though. Without something to threaten the young man with, the speaker can't get him to pay attention. Unless he can make the young man scared of old age, there's no way to force him to have kids.
This poem creates a primarily negative portrayal of old age, where the only possibility of joy and satisfaction is having something (in this case a child) that reminds you of your youth.