This scene does show Mr. Bennet at about his worst. He is lazy and neglectful. If he would really listen to Lizzy, he would have to act. Perish the thought! He is not thinking about his daughters' reputations, but about his own comfort.

The only thing I can say in his defense is this. He really doesn't expect Lydia will be any the worse for going. At home Mrs. Bennet is Lydia's prime example of ladylike behavior. Can Mrs. Forster really be that much worse? Maybe he hopes Lydia will get all her infatuation for officers out and come home ready to go back to being the rational (ha, ha) person she was before the regiment arrived. At least her bad behavior in Brighton will be away from the notice of all their neighbors, and maybe even from possible suitors for her sisters. What real evil can possibly come of it?