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The only reason I can think of waiting is because she might become a target if she was his wife however... I really don't see it as being any safer being his Fiancee if the goal of whoever wants to harm him (don't know if there even is) is to kill someone close, it would still be Usagi. The only other reason I can see is to prolong the manga, well guess I don't hate this manga so I wouldn't mind pulling off illogical plot developments.