The Unions of Elves and Men

[Morthoron]

Originally Posted by Huinesoron

Thinking on this a bit more, I suspect "doom" is used here to mean prophecy: it's not that Manwe made a whole new ruling at the birth of each of Elrond's children, but that they were already under the offer of a choice, and were prophesied to make that choice at the point Elrond departed overseas. Elrond's departure was a drawn-out process, so Arwen making the choice at her wedding fits into that.

Tolkien, Professor of Anglo-Saxon as he was, is using the word "doom" in the Old English sense, "judgement". Mandos makes an irrevocable pronouncement on the fates of those entwined. He does offer a choice to Elros and Elrond, and the children of Elrond, but again, once that choice is made it cannot be revoked unto the end of Arda.