What is the main theme of Shakespeare's Sonnet 116?

The topic of Sonnet 116 is love.  The poem is a rumination on love, if you will.  Stanza by stanza, here's a paraphrase:

1. Don't let me consider anything that would get in the way of a marriage between true minds.  Love does not change when its object's appearance or affections change, or if a lover turns or looks elsewhere, or is absent.
2. Love is like a star that guides a ship, a star that stays steady during great storms.  Love is the star that guides every wandering ship, a star whose value, quality, true nature is unknown even though its measure is taken to determine the location of a ship.
3. Love is not made a mockery of by time, it is not a fool or court jester.  Time, nor the grim reaper, will bring death to love.  Love lasts until the judgment day.
4. If my thoughts above are incorrect and it's proven to me, I've never written and no man has ever loved.

Love is as consistent and constant as the star ships use to navigate by.  It doesn't change even when its object stops loving or begins loving someone else, or is absent.  Love lasts until the final judgment.  And if all this isn't true, then the speaker has never written anything, and no man has ever loved.

The main theme of this sonnet, like so many of Shakespeare's sonnets, is love.  In the poem, he is talking about the constancy and permanency of love.

In this sonnet, Shakespeare talks about how love does not change.  He says love does not change depending on the circumstances.  He says it does not change over time.  He says it stays solid like a lighthouse.

He says that he is absolutely sure of these things -- if he is wrong, there has never been love in the world.

There really is not any other theme going on in this poem.

The theme of this famous sonnet concerns the transcendent nature of true love and how it overcomes any barriers or obstructions. True love, the speaker argues, does not change or alter with the passing of time, or with the fading of beauty and youth. Even though love is influenced by time, love is still more powerful than time, as the following quote proves:

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.

Time does of course impact beauty and the appearance of love's adherents, but it does not impact love itself, as true love remains constant "even to the edge of doom," or up until death itself, and, in some cases, beyond. In many ways, therefore the poem is a pageant to the power of true love and its constancy, which also serves a double purpose: the speaker, after all, professes this kind of love for his beloved, and the more successfully and poignantly he captures the might of love the greater his chance of being heard with favourable ears by the object of his affection.

What is the main theme of Shakespeare's Sonnet 116, and how does its structure enhance this theme?

The theme of the Shakespearean sonnet whose opening line is "Let me not to the marriage of true minds" (Sonnet 116) is the steadfastness and immutability of love. The first sentence, perhaps the most well known of the sonnet cycle, unambiguously establishes the thrust of the poem, the plosive p of "impediments" wielding the power of a fist striking a tabletop. One might observe that the remainder of the sonnet is a sequence of one-sentence restatements of the thesis, including several exquisite metaphors. The very brevity of these assertions contributes to their force.

Love seems to be perceived as a third party existing apart from, or in addition to, the lovers themselves. The concluding couplet states that the speaker is as certain of the truth of his argument as he is of his own existence: amo, ergo sum (I love, therefore I am). The phrase "nor no man ever lov'd" has an element of ambiguity, seeming to point toward two ideas: that anyone who has loved would agree with the speaker, and the possibility that anyone who has loved has fallen short of "the marriage of true minds."