Shakespeare Sonnet 116
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.
Note the bolded section of the sonnet. Here is where you can find the metaphor referring to the fixed nature of love.
Love is compared to a lighthouse (ever-fixed mark) which remains visible during a storm (tempest). Love is also compared to the north star (start), which remains fixed in the sky to guide ships (wandering bark).
You may find the attached website helpful. It takes Sonnet 116 and paraphrases each line.