The speaker spends the whole sonnet making what appear to be rather unflattering comparisons between the beauty of his lover and objects of nature such as coral, snow, and roses. Other poets may gush about how their mistresses' hair is like gold wire, but not Shakespeare. To him, these are completely false comparisons to make. He makes comparing your lover's walk to that of a goddess seem utterly ridiculous. Who's ever seen a goddess walk?

Yet despite the fact that the speaker's lover has breath less sweet than the perfumed scent of a rose, and despite the fact that, unlike a goddess, she can only walk upon the ground, the speaker wouldn't have it any other way:

And yet, by heaven, I think my love as rare
As any she belied with false compare.

William Shakespeare's "Sonnet 130" contradicts most poetry which speaks to the beauty of a woman. According to the speaker, his lover is incomparable to anything in nature. Nature is far more beautiful than the lover. 

The final two lines, or the ending couplet, continues to support what the speaker has claimed to this point. Although the lover is nothing like the natural phenomenons found in the world, she proves to be just as "rare." The speaker states his understanding that a women is not comparable to nature, for it is far too phenomenal. Illuminating this, the speaker also states that he understands that while other poets may tend to exaggerate the beauty of other women, women prove to be just as rare a beauty as the elements of nature. In essence, neither can be compared to the other. Each possess its (or her) own beautiful qualities.