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.
In other words, his lover is as rare and as valuable as any invoked by poets with false comparisons.