Moonstruck lovers moon over each other over — what else, but — moonlight dinners, and before you know it, what do you have, but a honeymoon. The honeymoon began with the ancient custom of drinking mead (sweet honey wine) at the wedding ceremony. This ritual then carried over to the first month of the marriage when the newlyweds drank a cup of mead each night, not only for its symbolic value (a reaffirmation of their vows), but also for its legendary aphrodisiacal qualities. The moon having gone full cycle during this time, the period was quite naturally called the honeymoon.
Dr. Samuel Johnson likened this time in the life of the new couple to the cycle of the moon, noting how its fullness was but a prelude to its decline. Dictionaries followed the good Doctor's lead, describing the honeymoon as "the period of waning affections." The honeymoon was now officially over.
For such cynics, a good and lasting marriage was a rare thing, happening only once in a blue moon. During the 16th century, no one in his right mind could believe that the moon was blue; hence the original phrase, read "Never in a blue moon."
Our powers of observation, however, increased significantly with the advent of the telescope, allowing us to note an occasional blue fringe. This enabled us to shift its meaning to "seldom" — lending "once" the requisite legitimacy.
A good and lasting marriage is also a rarity, but it doesn't mean you can't expect it. It is, after all, not like you were asking for the moon.