Lydia was a stout, well-grown girl of fifteen, with a fine complexion and good-humoured countenance; a favourite with her mother, whose affection had brought her into public at an early age. (9)

I agree Mr. Bennet should have listened to Lizzy because although he may not be privy to the realities of Jane’s or Wickham’s situation he knows enough of Lydia’s behavior to be cautious on that score alone since too young, too stupid and too inexperienced means little to him. Lizzy and Mr. Bennet think Lydia is silly and flirts every chance she gets: To Flirt - To act with levity; to be guilty of a kind of coquetry. (Johnson’s 1824). Not only is Lydia a flirt but in Ch. 41 Lizzy describes her as a flirt “in the worst and meanest degree of flirtation” in that she possesses a “rage for admiration” that excites contempt and censure in sensible people.

While there was an officer in Meryton, they would flirt with him; and while Meryton was within a walk of Longbourn, they would be going there for ever. (37)

I don’t think Lizzy or Mr. Bennet believe Lydia intends to sacrifice her virginity in Brighton. It is but one very bad possibility among many possibilities for trouble. Lizzy thinks Lydia’s non-stop flirting with anything in a red coat is an evil to them all; Jane’s lost hopes, affected by family impropriety, are evidence of it. Lizzy’s warning that Lydia, if not stopped will be known as the “most determined flirt that ever made herself and her family ridiculous” shows the she sees Lydia’s behavior as self-destructive to her own reputation and harmful to her sisters’ as well. I think Mr. Bennet’s consolation to Lizzy for her worries is revealing of his views on Lydia:

“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.” (41)

His consolation is also an admission that his youngest daughters’ behavior does not show them to advantage which ought to elicit more than amusement as their future depends on their marriageability. His claim “she cannot grow many degrees worse, without authorizing us to lock her up for the rest of her life” is said jokingly but is yet another admission Lydia’s behavior is harmful. Mr. Bennet knows Lydia will expose herself in Brighton and understands it is to her disadvantage as well as her sisters but apparently stops short of feeling it is lastingly harmful or destructive because only Lizzy & Jane ever try to check Lydia’s imprudent behavior (37).

I think Lydia’s “easy manners” (9), “high animal spirits” (9) and “wild giddiness” (41) that is uninfluenced by a proper sense of decorum coupled with her “youth… ignorance and emptiness of… mind” is a recipe for trouble but the danger must increase when she is given more freedom to govern the extent of her pleasures. There is a hint her moral instruction has not taken too well in a penchant to blaspheme (19, 23, 39) and add flirtations of the meanest degree and the following descriptions of temper and character and I think it is enough to create some worry in this reader at what trouble Lydia might get herself into or invite upon herself from others:

She was very equal, therefore, to address Mr. Bingley on the subject of the ball (9)

Miss Lydia's pressing entreaties that they [Denny & Wickham] would come in… (15)

Lydia, always unguarded and often uncivil, boisterously exclaimed… (23)

Lydia, self-willed and careless, would scarcely give them a hearing. They were ignorant, idle, and vain. (37)

The rapture of Lydia… Wholly inattentive to her sister's feelings… flew about the house in restless ecstacy, calling for every one's congratulations, and laughing and talking with more violence than ever… (41)

Mrs. Bennet… impressive in her injunctions that she would not miss the opportunity of enjoying herself as much as possible -- advice which there was every reason to believe would be attended to… (41)