Shakespears sonnet 116

Marriage is joining so the line "let me not the marriage of true minds" would mean let me not the joining of true minds.
 Impediments meaning obstacle the line "admit obstacles" with this the whole thing makes a lot more sense. Altering or alteration is to change so love is not love which changes when it finds something changed.
 The next line means the same thing. The line "it is an ever fixed mark that looks on tempests and is never shaken" that could mean it is unbreakable, never moving, and/or incredibly strong.
The next line, which says "it is the star to every wandering bark" is referring to love as a star to guide like the North star guided and guides sailors across the ocean at night.
No one knows what loves worth is but most know its importance. This poem speaks of loves strength, and importance.
In the end the poem says "if this be error and upon me proved I never writ, nor man ever loved." Shakespeare is saying my writing is nothing without love. And he seems to be challenging any one to prove him wrong, not that he wants to be wrong.