Sex Amongst the Maiar (and the Other Beings in Arda)

[William Cloud Hicklin]

Here it is:

Melian alone of all [the Maiar] assumed a bodily form, not only as a raiment but as a permanent habitation in form and powers like to the bodies of the Elves. This she did for love of Elwe; and it was permitted, no doubt because this union had already been forseen in the beginning of things, and was woven into the Amarth of the world, when Eru first conceived the being of his children, Elves and Men.

Reflecting a somewhat different idea at least wrt 'uniqueness' is this:

The least [of the Maiar] could have been primitive (and much more powerful and perilous) Orcs; but by practising while embodied procreation they would (cf. Melian) become more and more earthbound, unable to return to spirit-state

However, in this essay T (very much here 'thinking with the pen') then immediately rejected the idea of Maia-orcs having offspring, concluding that Eru would not create fear for them.

There is also a note to the Athrabeth, not specifically about Melian but connected, which posits that Luthien's becoming of man-kind, and Beren's return from death, were a great Exception granted in order that a strain of the divine would pass down among the Eruhini.