What's he saying?
"My mistress' eyes are nothing like the sun; / Coral is far more red than her lips' red;"
My mistress's eyes look nothing like the sun; coral is far more red than her lips are.
"If snow be white, why then her breasts are dun; / If hairs be wires, black wires grow on her head."
If snow is white, then her breasts are a dull brown (in comparison); if hairs are wires, then black wires grow on her head.
"I have seen roses damask'd, red and white / But no such roses see I in her cheeks;"
I have seen roses of pink, red, and white, but her cheeks are none of these colors;
"And in some perfumes is there more delight / Than in the breath that from my mistress reeks."
And some perfumes smell more delightful than the malodorous breath of my mistress.
"I love to hear her speak, yet well I know / That music hath a far more pleasing sound;"
I love to hear her speak, even though I know well that music has a far more pleasing sound;
"I grant I never saw a goddess go; / My mistress, when she walks, treads on the ground:"
I admit I have never seen a goddess walk, but my mistress, when she walks, steps (humanly) on the ground:
"And yet, by heaven, I think my love as rare / As any she belied with false compare."
And yet, I swear before heaven, I think she is just as extraordinary as any woman that may be described with false comparisons.
Why is he saying it?
Sonnet 130 is a pleasure to read for its simplicity and frankness of expression. It is also one of the few of Shakespeare's sonnets with a distinctly humorous tone. Its message is simple: the dark lady's beauty cannot be compared to the beauty of a goddess or to that found in nature, for she is but a mortal human being.
The sonnet is generally considered a humorous parody of the typical love sonnet. Petrarch, for example, addressed many of his most famous sonnets to an idealized woman named Laura, whose beauty he often likened to that of a goddess. In stark contrast Shakespeare makes no attempt at deification of the dark lady; in fact he shuns it outright, as we see in lines 11-12: "I grant I never saw a goddess go; / My mistress, when she walks, treads on the ground." Here the poet explicitly states that his mistress is not a goddess.
She is also not as beautiful as things found in nature, another typical source of inspiration for the average sonneteer: "My mistress' eyes are nothing like the sun; / Coral is far more red than her lips' red." Yet the narrator loves her nonetheless, and in the closing couplet says that in fact she is just as extraordinary ("rare") as any woman described with such exaggerated or false comparisons. It is indeed this blunt but charming sincerity that has made sonnet 130 one of the most famous in the sequence.
However, while the narrator's honesty in sonnet 130 may seem commendable, we must not forget that Shakespeare himself was a master of the compliment and frequently made use of the very same sorts of exaggerated comparisons satirized here. We even find them elsewhere in the sonnets, and in great abundance, too; note that while his "mistress' eyes are nothing like the sun," his fair lord's indeed are, as in sonnet 49: "And scarcely greet me with that sun, thine eye."
This may lead one to wonder, is it really pure honesty that the poet is showing in sonnet 130, or is there also some ulterior sentiment, perhaps that the dark lady is not deserving of the narrator's fine words? Or perhaps she is deserving but such words are not necessary, as though the narrator feels comfortable enough with the dark lady that he is able to show such honesty (which his insecurity regarding the fair lord prevents him from doing)? There are many ways to interpret how the poet's psychological state may have influenced stylistic choices in his writing, but these sonnets do not provide definitive proof.