Here's What Tolkien Himself Wrote About the "Intensity" of Elf Sex
J.R.R. Tolkien, unlike a significant portion of Middle-earth fanfic writers, did not care at all about writing about sex.
In fact, in the novels he personally completed and published (The Hobbit and The Lord of the Rings ), romance plays almost no role in the plot.
Aragorn and Arwen are a couple from the beginning. Eowyn's longing for Aragorn amounts to little, is shown to be a form of protest against her fate rather than actual love, and at most gives her an extra push to ride into battle, as she wanted to do anyway, and then she quickly pairs up with Faramir after the main conflicts of the book are resolved.
This spirit of chastity goes double for elves, who are near-perfect immortal beings, almost angelic in their state.
It is hard to imagine them doing something as earthly as sex...but they do.
After all, baby elves have to come from somewhere, and since elves are physically almost indistinguishable from humans - while since Peter Jackson's films everyone imagines Tolkien's elves to be pointy-eared, Legolas is mistaken for a human at least once in The Lord of the Rings - their appearance must logically involve sex.
And it seems that Tolkien did give some thought to the question of the sex of elves in his voluminous notes, the compilation of which allows his descendants to publish new books in his name to this day.
This particular note was published in The Nature of Middle-earth, which came out last year, and a Redditor was kind enough to post an excerpt for those who were concerned with the question (purely for the sake of deeper lore understanding, of course!):
"…the act of procreation, being of a will and desire shared and indeed controlled by the fëa [soul], was achieved at the speed of other conscious and willful acts of delight or of making. It was one of the acts of chief delight, in process and in memory, in an Elvish life, but its intensity alone provided its importance, not its time or length: it could not have been endured for a great length of time, without disastrous "expense"…it is longer and of more intense delight in Elves than in Men: too intense to be long endured. "
So basically, elf sex is supposed to be quick, but intense enough that if it took any longer, it would have posed a "disastrous" risk!