Lie Back and Think of Rivendell

In Leviathan, Thomas Hobbes raises the important question of elf reproduction avant la lettre when he opines that Adam and Eve could not have been, as it were, shagging until after they had eaten of the forbidden fruit. Before they had lost eternal life, they could not have known sex, 'For if immortals should have generated, as mankind doth now; the earth in a small time, would not have been able to afford them place to stand on.'

Now, there is an important caveat to this point, which is that while Adam and Eve had eternal life, they were in a non-Malthusian metaphysical space - they were not subject to resource constraints. So Hobbes is right with regard to this narrowly bounded scenario. If Adam and Eve were immortal, and simply procreated in the normal way, in the fullness of time there would be more Cains and Ables than atoms in the universe.

Elves, who are actually immortal (at the 'hard' Tolkien level) in terms at least of longevity, are unlike Adam and Eve presumably subject to Malthusian forces. But still, they likely face a less extreme version of this problem. If you live for a very long time, or indeed forever, you can make an awful lot of babies. And this causes headaches for reasons beyond the crying and loss of sleep. Not only would it mean overpopulation. It would also cause severe social problems with regard to the matter of inheritance - imagine the disputes that would arise over wills and probate when Great-great-great-great-great grandpa Finion is killed by a balrog, leaving behind ten thousand heirs.

Elves then presumably have ways of ensuring that they produce very few young. A range of possibilities present themselves, with varying degrees of interest/gameability:

1. They have sex, but not in a procreative way, if you catch my drift. This may be a productive idea for generating erotic fiction, but is not I think a particularly interesting thing to explore via the medium of D&D (though, as ever, your mileage may vary).
2. They have sex on rare occasions and this is perhaps timed to coincide with phases of the moon, alignments of planets, particular weather events, passing comets, etc. Totally I think gameable: imagine a campaign setting in which elves only get to have sex once every year at the time the first hurricane makes landfall at Saxinfraxin, and in order to do so every elf in the world has to travel back to a particular spot to find a mate.
3. For two elves to have sex, they need for ritualistic (or perhaps even spiritual or biological) reasons to be in the possession of a rare type of jewel, flower, metal, and so on. There is naturally huge demand for the material in question and a cottage industry of (human) adventurers and pioneers who go out into the dangerous places of the world to procure it.
4. Elves practice infanticide and child sacrifice at vast scale. This is dark. But fits nicely with my preferred conceptualisation of elves as inscrutable and unflinching Noldor/fae/Melniboneans/Eldar rather than Dragonlance style qualinesti types. And it would naturally generate interesting possibilities for adventure. (Idea for a Fantasy Novel No. 16,789: Human father of half-elf progency goes to the great elf city to rescue his infant child from sacrifice. Not bad, eh?)
5. They chiefly have sex with humans, safe in the knowledge that this will produce short-lived (to their eye) half-elf progeny. And they save sex with each other for special occasions. This sounds vaguely like the plot of one of those 'dark fantasy' novels you see on the high shelves in WH Smith - the elf who falls in love with the human he/she thought was there for mere pleasure - but there are more interesting directions to take the idea. What if, for example, having a half-elf child is thought of as a special honour or even of religious significance? And, if this practice is very common and widespread, what kind of cultural expectations, social conventions, and conflicts arise around the presence of so many half-elves in human society? 

You may have your own ideas.