"A woman's face with Nature's own hand painted / Hast thou, the master-mistress of my passion;"
Nature painted you with the face of a woman, you master and mistress of my passion;
"A woman's gentle heart, but not acquainted / With shifting change, as is false women's fashion;"
You have the gentle heart of a woman, yet you are not fickle like so many changeable women;
"An eye more bright than theirs, less false in rolling / Gilding the object whereupon it gazeth;"
Your eyes are brighter than women's, but not as deceptive as theirs; you shed golden light upon any object you gaze upon;
"A man in hue, all 'hues' in his controlling / Much steals men's eyes and women's souls amazeth."
A man of your appearance sets the standard for what a man should look like; your beauty attracts the eyes of men and amazes the souls of women.
"And for a woman wert thou first created; / Till Nature, as she wrought thee, fell a-doting,"
And you were first created to be a woman, but Nature fell in love with you (or made a mistake) as she was crafting you,
"And by addition me of thee defeated / By adding one thing to my purpose nothing."
And defeated me by adding one thing to you, a thing that does not aid my goal.
"But since she prick'd thee out for women's pleasure / Mine be thy love and thy love's use their treasure."
But since she chose you to be for women's pleasure, your love will be mine, yet the use of your love is for women's benefit.
is almost as though the narrator is saying all this with the ulterior motive of justifying his own attraction to the fair lord. Scholars are divided over what this attraction really equates to, but the prevailing view is that although the attraction is certainly present, this does not necessarily imply that it is sexual. In lines 11-12, for example, the poet explicitly bemoans the fact that the fair lord was created as a man, but at the same time he explicitly denies any interest in the fair lord's genitalia: "And by addition me of thee defeated / By adding one thing to my purpose nothing." That "thing" is presumably the fair lord's penis, following common Shakespearean wordplay.
In the sonnet's closing couplet - tying in with the theme of platonic love vs. carnal lust - the poet concedes that the fair lord's love can belong to him even as the use of his love (that is, the sexual act) remains for the ladies. Note the poet's pun on the word "prick" in line 13: as a verb it can mean "to choose," while as a noun it can be a vulgar term for "penis." Finally, note that sonnet 20 is the only of Shakespeare's sonnets to use exclusively feminine rhyme - that is, end rhymes of at least two syllables with the final syllable unstressed - perhaps a deliberate attempt to further feminize the fair lord.
For a good example of the kind of creativity used by interpreters of the sonnets, let us consider the position held by some scholars that the poet intentionally encrypted the actual name of the fair lord into the lines of sonnet 20. Support for this hypothesis comes from the fact that the letters HEWS (with U at times in place of W) appear in every line in the sonnet but one; also note the "hue" and "hues" in line 7 (this second instance italicized in the Quarto), and the assonating "use" in line 14. Some take this as evidence for a Mr. Hughes as the true identity of the fair lord. Others see the letters as the poet's initials (WS) plus the first two letters of either Henry or Herbert (HE), possibly resorting to these names since the first letter of William or Wriothesley was already being used. One might even go so far as to claim that Shakespeare's use of the word "wrought" in line 10 was a deliberate alliterative reference to Wriothesley, or that the poet numbered the sonnet in accordance with the fair lord's age (Herbert would have turned 20 in 1600, Wriothesley in 1593). Obviously such interpretation is highly speculative and must remain inconclusive without corroborating historical evidence. But readers can enjoy wondering whether any of these ideas is true.