Look in thy glass and tell the face thou viewest
Now is the time that face should form another;
Whose fresh repair if now thou not renewest
Thou dost beguile the world, unbless some mother
For where is she so fair whose uneared womb
Disdains the tillage of thy husbandry?
Or who is he so fond will be the tomb
Of his self-love, to stop posterity?
Thou art thy mother's glass and she in thee
Calls back the lovely April of her prime;
So thou through windows of thine age shalt see
Despite of wrinkles, this thy golden time
But if thou live, remembered not to be
Die single and thine image dies with thee
The “But” introduces a new thought. The speaker asserts that the young man may be prepared to live but not to be remembered; then his image will also die.  Dying childless essentially will erase him and his beauty from human memory.
Sonnet 3