I always thought the line was "let me not...admit impediment". But the Oxford edition on the web says:

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,                 5
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;         10
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 prov'd,        
I never writ, nor no man ever lov'd.