Tolkien answers this in letter #153:
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Túor weds Idril the daughter of Turgon King of Gondolin; and 'it is supposed' (not stated) that he as an unique exception receives the Elvish limited 'immortality': an exception either way. Eärendil is Túor's son & father of Elros (First King of Númenor) and Elrond, their mother being Elwing daughter of Dior, son of Beren and Lúthien: so the problem of the Half-elven becomes united in one line.
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I don't see Tuor as a coward for rejecting Death, I think he rather chose to stay with Idril. But as stated in another letter, it may not have the best decision the couple made as the Gift of Men would eventually be longed after even by the Valar.