Before letting this issue go, I want to take one last shot at making sense of the matter. Rather than trying to interpret snippets of text, I'd like to look at the issue dispassionately based upon lineage and parentage, as, presumably, the Valar would have needed to look at the issue when they decided it at the end of the First Age.
Beren was a Man. Luthien, despite her lineage by birth, received leave to be counted as mortal and returned to Beleriand with Beren... "for a time". Both were mortal. Dior was a Man, not a Half-Elf. Tuor and Idril begot Earendil, a true Half-Elf. Dior wedded Nimloth, an Elf, and begot Elwing, a Half-Elf. Elrond and Elros, sons of Earendil and Elwing were true mixed bloods, and were given leave to choose. Elros chose to be a Man and married... whatever her name was, a Man, and begot Mannish children and descendents. Elrond, who chose to be an Elf, married Celebrian, an Elf. Their Children are... Elves. Arwen is given the choice of Luthien and chooses to be numbered among the Men. Elladan and Elrohir have no such choice, they are Elves.
Pick the logic apart people!
