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"Lydia will never be easy until she has exposed herself in some public place or other, and we can never expect her to do it with so little expense or inconvenience to her family as under the present circumstances." (43.15)
And that, Mr. Bennet, is where you're wrong. He thinks that sending Lydia to Brighton to get her kicks out will minimize the damage, but it actually ends up almost destroying the family. Good thing Darcy steps in to save the day—and to keep the family respectable enough to marry into.
"If you, my dear father, will not take the trouble of checking her exuberant spirits, and of teaching her that her present pursuits are not to be the business of her life, she will soon be beyond the reach of amendment. Her character will be fixed, and she will, at sixteen, be the most determined flirt that ever made herself or her family ridiculous; […] Oh! my dear father, can you suppose it possible that they will not be censured and despised wherever they are known, and that their sisters will not be often involved in the disgrace?" (43.18)
Mr. Bennet thinks that Lydia's behavior isn't going to reflect badly on Lizzy or Jane, but he's wrong. Maybe if he ever left his library to supervise his daughters at one of those balls where they make themselves ridiculous, he'd actually know something about how the world works.
"Do not make yourself uneasy, my love. Wherever you and Jane are known you must be respected and valued; and you will not appear to less advantage for having a couple of—or I may say, three—very silly sisters." (43.20)
Just one more example of how little Mr. Bennet understand about the way family works. It's like a nightmare version of the Three Musketeers, you know, one for all and all for one: what Lydia does reflects badly on the entire family. (But we have to ask: if Kitty and Mary had been sensible, would Lydia's personality have mattered less? Could four good reputations outweigh one bad?)