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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."
"If you were aware," said Elizabeth, "of the very great disadvantage to us all which must arise from the public notice of Lydia's unguarded and imprudent manner—nay, which has already arisen from it, I am sure you would judge differently in the affair." (41.15-16)
Huh. So both Elizabeth and Mr. Bennet kind of know all along that Lydia's whole Brighton adventure is going to end in nothing but trouble. It's interesting that Mr. Bennet's approach to raising his daughter is one that is probably more often used for boys (since his theory is that Lydia needs to sow wild oats at some point in her teenage years). Elizabeth, on the other hand, has a little more perspective on the fact that, in their society, what would be water under the bridge for boys would mean social annihilation for girls.
If gratitude and esteem are good foundations of affection, Elizabeth's change of sentiment will be neither improbable nor faulty. But if otherwise—if regard springing from such sources is unreasonable or unnatural, in comparison of what is so often described as arising on a first interview with its object, and even before two words have been exchanged, nothing can be said in her defence, except that she had given somewhat of a trial to the latter method in her partiality for Wickham, and that its ill success might, perhaps, authorise her to seek the other less interesting mode of attachment. Be that as it may, she saw him go with regret. (46.24)
This is one of the few times that the narrator just comes right out and tells us what principle of life is being debunked. Here, for example, it's the idea of love at first sight, which is clearly for Austen a far inferior idea than don't judge a book by its cover.
[Mary:] "Unhappy as the event must be for Lydia, we may draw from it this useful lesson: that loss of virtue in a female is irretrievable; that one false step involves her in endless ruin; that her reputation is no less brittle than it is beautiful; and that she cannot be too much guarded in her behaviour towards the undeserving of the other sex." (47.44)
Here, Mary is clearly echoing the sort of horrible, formal advice given to young ladies by the conduct books (books about how to behave and why) being published at the time. The awful humor here is that, of course, in the actual situation of the Bennets, with actual human beings, with real feelings involved, no one wants to hear this unsympathetic nonsense about the purity of women.