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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)
Mr. Bennet is basically saying that Lydia is destined to humiliate herself, and that going to Brighton is a great opportunity for her to do it without costing him too much money and inconvenience. Do we have to spell out what we're thinking? Mr. Bennet isn't being a super-good parent here. His lax attitude comes back to bite him in the derriere, too, when Lydia runs off with Mr. Wickham.
"Indeed you are mistaken. I have no such injuries to resent. It is not of particular, but of general evils, which I am now complaining. Our importance, our respectability in the world must be affected by the wild volatility, the assurance and disdain of all restraint which mark Lydia's character. Excuse me, for I must speak plainly. 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)
Elizabeth would make a great soccer mom someday. She has more sound parenting instincts than her father. And the thing is, Elizabeth already knows that Lydia's behavior <em>has</em> hurt her sisters. She's heard first-hand from Darcy that he separated Bingley from Jane in part because of the young Bennet girls' wild behavior. Elizabeth also accurately predicts the events to come. When Lydia does go too far (and runs off with Wickham) folks like Lady Catherine and even the odious Mr. Collins want nothing to do with the Bennets. They go so far as to say <em>all </em>of the Bennet girls are now ruined.
"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. We shall have no peace at Longbourn if Lydia does not go to Brighton. Let her go, then. Colonel Forster is a sensible man, and will keep her out of any real mischief; and she is luckily too poor to be an object of prey to anybody. At Brighton she will be of less importance even as a common flirt than she has been here. The officers will find women better worth their notice. Let us hope, therefore, that her being there may teach her own insignificance. At any rate, she cannot grow many degrees worse, without authorizing us to lock her up for the rest of her life." (43.20)
Mr. Bennet is making a decision to allow Lydia to experience failure and learn the extent of her silliness. You could argue, based on this passage, that Mr. Bennet is a good father, but we disagree heavily. We think that his other actions (or lack thereof) reflect the opposite. He's also not being very protective of Lydia. He clearly hasn't taken into account that there are men out there (ahem, Wickham) who will prey on poor girls just for fun. It isn't only wealthier girls who are in danger.