> ...there’s nothing in the Legendarium suggesting there can be unions of anything other than Men and Elves, Men and Orcs, and Elves and Maiar.

Tolkien, Race, and Racism in Middle-earth[1] has a long section discussing this. Tolkien regarded elves, men, hobbits, dwarves, and orcs all as races of one species, who thus could interbreed.

[1] https://www.amazon.com/Tolkien-Racism-Middle-earth-Robert-St...

If you are interested in controversies over issues of race in Tolkien's work, I've just started reading a new book that seems quite good, Tolkien, Race, and Racism in Middle-earth.[1]

Most writing I see on this is no better than what you would expect in a fanzine or on Reddit, but this author is a historian (of French Marxism) and his work seems very measured and thorough. So far in my reading, he seems more defensive of Tolkien than not.

[1] https://www.amazon.com/Tolkien-Racism-Middle-earth-Robert-St...