Sonnet 116 suggests a "marriage of minds," a true, or spiritual love beyond the physical. However, taken in context with the sonnets before and after it, Number 116 appears to be less about a Platonic Ideal of Love, and more about the implications of loving unconventionally.
The prevalent images in the sonnet refer to nautical themes; "Ever fixed mark" refers to a beacon that's not altered by storms; "Star" may refer to the North Star, which never moves relative to other stars and is used to guide ships, particularly to the reference to "taking his height," or measuring its angle above the horizon to determine a ship's position. Such are the metaphors for this unaltering true love. Love endures beyond the physical (rosy lips and cheeks may be cut down by time) Love doesn't change over time, but endures forever. The couplet makes the final point that if this is not true, no one ever loved, and Shakespeare never wrote anything.
For a line-by-line analysis, see the link: