Shakespeare's Sonnets. Sonnet 3

Sonnet III

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 unear'd 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 shall see
Despite of wrinkles this thy golden time.
But if thou live, remember'd not to be,
Die single, and thine image dies with thee.

The first 126 sonnets of Shakespeare are known as the “Fair Youth Sonnets.” Shakespeare has addressed all of these to a ‘fair youth’ and the bond and deep love of Shakespeare for this young man is apparent in all of them.

My understanding of Sonnet 3 - When you look in the mirror and you see yourself, remind yourself that the youthful looks that stare back at you will not remain. The only way in which you will be able to preserve any semblance of them will be by passing them on to your child.
So, stop being vain, you are not only denying yourself this happiness, you are also cheating some mother-to-be of the joys of motherhood as are you bilking the world too.
Your mother can look at you and proudly recall how beautiful she used to be when she was young. So, while you still can, make a child, who will carry on your legacy; and when you are old you can see your image in your off-spring, and relive and remember your youth, just as your mother does with you. But if you remain conceited and choose not to procreate, when you die, your image will die with you, and with it, a worthy, beautiful lineage will be lost to the world forever.