Like Mrs. Weasley said in the 6th book, everyone is eloping because they think they're going to be dead the next day, which is what happened in their case. It sounded, by Tonks' description, like they had eloped since it was such a small ceremony...but it was like, 3 weeks after Dumbledore's death, which was a bit rushed. I know it was all sweet and romantic that they died together, but it angered me a bit that they didn't think about the baby before running off into the battle...poor Teddy is now subjected to Harry's fate, even if he does get a happier childhood than Harry ever did.