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I don't think there was a union between Elves and Men in Numenor, particularly because after a certain time there came jealousy of Men to Elves and their immortality. As for "beauty", I daresay Numenoreans were as fair as the Elves, especially Erendis, who on a certain passage on the UT was described just as beautiful as the Elven company who graced her wedding with Aldarion (can't quote it though, I haven't got my book).
Idril, the mother of Earendil and the wife of the mortal Tuor, passed out of Middle-Earth after Gondolin was destroyed. About Mithrellas, she disappeared not only from ME but also from (as far as I know) Tolkien's works--and Imrazor was just a bit weird. In the UT I think there is something written about him harboring her and then taking her to wife, but no more is said after she disappeared.
Elves don't marry mortals, with the obvious exceptions. Luthien became mortal because Iluvatar through Mandos and Manwe gave her a choice of mortality without certitude of happiness with Beren or bliss in Valinor forever without him. He gave her this choice because she touched Mandos and Mandos asked Manwe who asked Iluvatar.
TO understand fully about Arwen, let's recall that Earendil the Half-Elven, his wife Elwing, and his sons Elrond and Elros were given a choice on which kindred they would belong. As told in the Silmarillion, only Elros chose to be mortal, and the other three were classified and "judged" as Elves.
Arwen became mortal because she and her twin brothers were given a choice as well--to stay in Middle-Earth even after the departure of Elrond would mean they would soon die; but if they left for Aman, they would be immortals. I do not exactly know what happened to her brothers, but since Arwen stayed in Middle-Earth and because of her choice to become mortal as well, she died.
I hope this enlightens you as well...
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