My youngest brother and his girlfriend, who have said for a long time that they were planning to get married, just got formally engaged, picking a date right after they graduate in 2012. "Their" song is "Smile" by Uncle Cracker. Obviously, a song about being in love: ("You make me dance like a fool, forget how to breathe...) and yet not an obvious wedding song because it's about the giddiness of love, perhaps new love, without bringing in the more serious aspect of commitment. And bro and FSIL are serious and mature about their commitment.