Shakespeare's "Sonnet 116" is one of his most often quoted, especially as part of marriage ceremonies, and the sonnet actually begins with a ceremonial tone and focuses on an unusual element:
Let me not to the marriage of true minds/Admit impediments.
In the context of a marriage in which love usually appears as the central focus, Shakespeare centers on the "marriage of true minds," not only implying that intellect has played a part in the decision to marry but also that the partners are equal. He quickly shifts to a more conventional aspect of marriage, but the opening lines are meant to establish that the intellect is as important as the heart.
The theme in the following six lines is that love is not true love if it attempts to change the other partner--true love honors the individual and does not try to mold the individual's behavior. More important:
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.
Using a metaphor that anyone in English society (as a sea-going nation) would understand, Shakespeare compares love to a lighthouse that guides any lost ship, whether that ship be particularly valuable or not, and love that is not affected by "tempests."
In the last four lines, Shakespeare's imagery is very conventional in the context of many of his sonnets: he argues that love is never affected by time:
. . . 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.
In other words, just as love is like a lighthouse, fixed and permanent, love remains constant and is not altered by time and death, lasting until Judgment Day.
The concluding couplet is Shakespeare's challenge to those who might argue with his sentiments in this sonnet: if I am wrong about this, then I'll acknowledge that I've never written anything and that I've never loved.