Actually, after thinking about this issue some more, I'm really surprised that they're going through with it. Like all teenage pop acts, Brittany Spears and N-Synch are largely about style and image, rather than substance. Getting married could hurt both of their images more than the time when Hugh Grant got caught getting head from a prostitute. It'll especially hurt Justin, since image is all most boy bands have. Brittany will have to drop the schoolgirl act, since that won't really work for a married woman, but asides from that she should be O.K.
Ever see the episode of the Simpsons in which Homer is part of a very successful singing "barber shop" quartet, and his publicist tells him that he has to lie about being married because that way "women can think that they can have sex with him". With boy bands, this constitutes the majority of the appeal. You can shave twice a day to avoid a stubble look. You can lie about your age. You can dress in all the A&F clothes you want, but when you get hitched, people aren't going to see you as a "boy" anymore. It's gonna be hard having a boy band with a married dude in it, or - 6 months from now - a divorced dude