http://lotrscrapbook.bookloaf.net/ref/rohanletters.html
Tolkien states the Rohirrim are not medieval. I think we run a terrible risk in assuming all aspects of culture to be identical to that of the Dark Ages, and highly doubt that a couple of Degas' and Lin's rank would be year-and-day handfasted, anyway. Nor does that happen without the interest of both families. Besides, Tolkien speaks of no couple in his histories who had children or were expecting before the fact, even among the Hobbits, who were probably the most "earthy" beings on Middle Earth. Remember, Elempi dear, that you protested hints that Saeryn might have taken a "tumble in the hay". Private trysts, I truly believe, would not be appropriate.
I am sorry, Bethberry, but I have no access to the Letters. Could you provide a synopsis of the information you're referencing?