…I think my love as rare As any she belied with false compare (line 13)
This is the payoff for all of this criticism of his mistress. He has shown us in this sonnet that chasing after some made-up ideal will only make you miserable. In fact, if you compare women to white snow or red coral, then you have belied, or misrepresented, them. He loves this particular woman even though she can't begin to measure up to the perfection of a goddess.