Leaving aside the billions the affair would generate for gossip columns, the possibility of posthumous marriage would wreak havoc for estate planners. What would be the inheritance rights of a posthumously marrying spouse? What level of proof is required to show that s/he was actually the fiance(e) of the decedent? What if the decedent had planned to sign a pre-nuptial agreement limiting the new spouse's inheritance rights? What if everybody knows the decedent would have backed out of the engagement prior to the wedding ceremony?
In France, legal scholars have the pleasure of tangling with such questions, as illustrated by the recent application of the fiance of a man who died as a result of "Muslim" fanatic Mohamed Merah's deranged shootings.
No such lurid details exist in this case as posited in the Anna N. Smith hypothetical above. Rather, the bereaved fiancee, carrying the decedent's child has petitioned for a respectful ceremony in which she can posthumously wed her untimely dead betrothed.
States local attorney arranging the matter, "it's a really moving ceremony, with an empty chair representing the dead spouse."
And as to the questions of proof and legitimacy of the engagement? In France, apparently at least some of those problems are dealt with by a curious level of constitutional authority given to the executive. No less than the president must sign off on the application to posthumously wed.