deeply amazed by his beauty. His beauty even affects their very souls.
This sonnet also has a structure with a division between the octet and sestet. In the octet the speaker praises the beauty of the young man and compares and contrasts him to beautiful women. In the sestet the speaker explains how Nature decided to make her creation a man instead of a woman.
The sestet begins (in line 9) with the speaker declaring that Nature had originally intended her creation to be a woman. But then Nature was so overcome by the beauty of her own creation that she fell in love ("fell a-doting") with it. So, Nature made one "addition" or change to her creation: she turned it into a man. This is great praise of beauty indeed. The speaker is saying that the young man is so beautiful that even the creation goddess fell in love with him.
Of course, for the speaker, who is male, the addition or change made by Nature was not a good change for him: "to my purpose nothing" (line 12). The speaker is stating that a beautiful man does nothing for him sexually. Although he appreciates the young man's beauty and loves the young man platonically, he regrets that this particular creation by Nature was not a woman.
The speaker, however, good-naturedly accepts the situation, as he comments in the couplet. Since Nature decided to create the young man for women's sexual pleasure (line 13), the speaker will accept having a platonic love with the young man