What does William Shakespeare mean by "If this be error and upon me proved, I never writ nor no man ever loved"?

These lines are from the couplet of Shakespeare's sonnet 116, Let me not to the marriage of true minds ...

Let me not to the marriage of true minds
Admit impediments. Love is not love
Which alters when it alteration finds,
Or bends with the remover to remove:
O no; it is an ever-fixed mark, 
That looks on tempests, and is never shaken;
It is the star to every wandering bark,
Whose worth's unknown, although his height be taken.
Love's not Time's fool, though rosy lips and cheeks 
Within his bending sickle's compass come; 
Love alters not with his brief hours and weeks, 
But bears it out even to the edge of doom.
   If this be error and upon me proved,
   I never writ, nor no man ever loved. 

The last two lines serve as an affirmation to the truth of what the speaker says throughout the poem.

The sonnet's theme is the nature of true love. The speaker states in the first two lines that he will accept or admit that there is, in the union of those who share a similar sentiment, i.e. love, any hurdle or barrier to taint such a love.

To confirm this statement he says that true love is unalterable and does not change, either because of circumstances or that the ones who so love are transformed in some way or another. Such love is permanently fixed and does not alter even when death (the remover) or some other unfortunate circumstance either removes or attempts to remove the one who is the object of such love.

This love can withstand any storm as a lighthouse does and retains its position. He uses a metaphor equating this love to a star that provides guidance to any ship that might have lost its way. Its true value can never be measured even though one can, in a literal sense, measure the height of a lighthouse or even guess at the distance of a star.

Furthermore, a love like this is not affected by the vagaries of time (equated here also with death) even though one may age and lose luster and vitality. This love does not adapt when times change - it is forever constant and it is able to survive until the end of time itself.

Finally, in the rhyming couplet, he states that if he is mistaken in his belief and his error is proved, then he has never written and no man has ever loved. This, of course, emphasizes the truth of what he believes for, he has written and men (and women) have loved. On this basis then, it is impossible to challenge his opinion.