THEY that have power to hurt and will do none,
That do not do the thing they most do show,
Who, moving others, are themselves as stone,
Unmoved, cold, and to temptation slow--
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This takes up the same argument as sonnet I, and indeed it is a sonnet that can hardly stand on its own because 'if all were minded so' implies that we know already how the youth is minded to behave, information which we only derive from what has gone before. ...
... 'Youth rapidly wanes, but this decrease may be made up by the children that a young man may beget. Otherwise all are doomed to age and decay. Since you are manifestly so beautiful, let the fate of dying out be left to barren, harsh and sullen souls. You are so well endowed by nature, that it is clear she intended you to be a seal, from the impress of which many copies should be made'. shakespeares-sonnets.com/
Such a powerful argument and message wrapped up in this superb sonnet.