I don't plan on getting married, but I like Kimi + Boku = Love by TegoMasu used in Lovely Complex. It's not something to be played during the ceremony, but maybe during the after-party because it's catchy... :)
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If for the actual ceremony, I'd love to hear the piano version of Sakura Kiss from OHSHC.
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This is the original version by Kawabe Chieko
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