LIBBY: Oh my God, is this [Dean] your escort?
RORY: Yeah, it is.
LIBBY: You are totally getting married.
Obviously, idiot characters like Libby cannot be trusted to give accurate predictions. Although Rory and Dean do get symbolically married at the ball itself.
Note that Emily told Lorelai the girls needed elbow-length white kid gloves for the ball, but they are clearly wearing elbow length white satin gloves instead. Honestly, without Emily supervising everything personally, this ball is going to pot. Also, Emily obviously managed to get Rory to put her hair up.
The Annotated Gilmore Girls 