Actually, it's a myth not backed up by documented fact.
We have an
entire board dedicated to the History Of Mead. Here is a
thread from Dan McFeely regarding this very subject in the child board
"Mead Legends and Tales."
Nobody has posted there for a while but there is are some great discussions in there if you are curious.
Well, wikipedia lists it too and I wouldn't dismiss it as myth so quickly. Here is a copy and paste of some of what they say that's related.
C & P
In many parts of Europe it was traditional to supply a newly married couple with enough mead for a month, ensuring happiness and fertility. From this practice we get honeymoon or, as the French say, lune de miel[4][5]
End C & P
The [4][5] reference these publications to back that up.
Gayre, Robert (1986). Wassail! In Mazers of Mead. Brewers Publications - Boulder, CO. ISBN 0-937381-00-4. , p.22
Acton, Bryan (1968 ). Making Mead. The Amateur Winemaker. SBN 900841-07-9. , p.14
There are other references to this tradition as well and at the very least, they say it's a theory, but I haven't read any reference to a real published publication that said it was disproven as myth, just opinion. I'm not so quick to jump on the myth train.
BTW, I apologize for posting the same subject as a previous poster. It did not show up in my search.