Elf sex
Catherine O points to a fascinating article on "What Tolkien Officially Said About Elf Sex":
Ever since the movie of . . . Fellowship of the Ring came out, there seem to be two popular ideas about Elves’ sex lives. Either they are radiantly asexual, or they are all screwing each other madly, along with any dwarves, hobbits, and men who happen along. Whichever you prefer is usually based on how attractive you think Orlando Bloom is.
More specifically:
The good news is that elves like sex. . . . The bad news is that elves tend to lose interest in sex after they’ve had kids.
It's been noted that Legolas never married. Make of that what you will.
Oh, I should note that the article makes a couple of comments to the effect that the elves represented Tolkien's "ideal people, based on his own values," but doesn't actually provide evidence for that; if Tolkien said so, then fine, but the article doesn't convince me of it. This paragraph brought to you by the Society for Not Ascribing Philosophies Written in Fiction to Their Authors (SNAPWiFTA).
One more thing: from near the end of the article, good news for slash writers:
Although Tolkien never said that the elves DID have hot gay sex, he also never said that they DIDN’T.
The author of that article has also written a longer and more serious analysis of sex in Tolkien's writing, which I haven't read yet.