I have found this:
The word honeymoon has its roots in the Norse word "hjunottsmanathr" which was anything but blissful. Northern European history describes the abduction of a bride from neighboring village. It was imperative, that the abductor, the husband to be, take his bride to be into hiding for period of time. His friends assured his and her safe keeping and kept their whereabouts unknown. Once the bride's family gave up their search, the bride groom returned to his people. This folkloric explanation presumably is the origin of today's honeymoon, for its original meaning meant hiding.
Also, this:
Historically, the groom's father would provide the first month's ("moon") worth of mead- an ale made from honey.
Hence, Honey moon.
And also this:
Babylon - In order to increase the virility and fertility of the newlyweds, the father of the bride would provide his son in law with all the mead (a honey-based drink) he could drink during the first month of the marriage (and therefore "moon").
Also, many other theoies. So my conclusion is: no one knows for sure.