Explain the point Shakespeare makes in the first twelve lines and the rhymed couplet of this sonnet.
I believe Shakespeare was explaining the contrasts of his lover to nature; not in the sense of saying she did not have beauty, but that there are more beautiful, tangible things in nature than his lover.
The point that William Shakespeare makes in the first twelve lines and the rhymed couplet of Sonnet 130 is that his lover is uniquely beautiful in her own way - even if he and/or others do not see her beauty as being comparable to the beauty of some other things in life.
Shakespeare states that the physical attributes of his mistress do not compare to the grandeur of the sun or to the richness of red coral. In addition, he states that snow has a certain beauty that exceeds her beauty. In fact, he compares his lover's hair to black wires - hardly a rousing endorsement of her locks!
He also states that her cheeks do not exhibit the color of roses. Furthermore, he states that some fine perfumes have a better aroma than his mistress' breath.
Shakespeare continues with these comparisons of his lover to other things in life, such as music sounding better than his lover's voice. He alludes that he understands that goddesses probably walk with a light air, but that his mistress...
when she walks treads on the ground.
... as if she's just plodding along.
However, in the final concluding couplet, Shakespeare conveys the fact that he believes his lover is a rare woman, with a beauty original to her. He states that no one can compare to this singular, unique beauty and that is why he loves her and why she is his and no one else's. The poem is a testament to the uniqueness of each individual - how we all have our own style and beauty - and how beauty truly is in the eye (and mind) of the beholder.