Sex = Marriage, Even On the Run

Tolkien describes elvish marriage in detail in the LACE essay. What he states about sex and marriage...well, I can't do any better than to quote him directly. He notes that, among his fantasy people in Middle-Earth, "Marriage is chiefly of the body, for it is achieved by bodily union, and its first operation is the begetting of the bodies of children, even though it endures beyond this and has other operations. And the union of bodies in marriage is unique, and no other union resembles it."

It follows from this that, among the Elves, "It was the act of bodily union that achieved marriage...it was at all times lawful for any of the Eldar, both being unwed, to marry thus of free consent one to the other without ceremony or witness…in flight and exile and wandering, such marriages were often made." Tolkien then proceeds immediately to a discussion of begetting of children. So, "marrying thus of free consent" means that the elf-man and elf-woman involved agree to be life partners, and that there is no excuse for elf casual sex.

Then again, Tolkien does mention that Celeborn was "the lover of Galadriel, who she later wedded." This comment does date back to an early set of notes when Celeborn's name, in Quenya, was Teleporno. You can see why he changed that one. (History of Galadriel and Celeborn, UF)