Why on earth would Mr. Bennet (or Mrs. Bennet or Jane or Lizzy or anyone else) think Lydia would be so totally depraved to throw away her virtue (if that's what people are suggesting)? That's a lot different than flirting and making a spectacle of oneself.

I do not condone Mr. Bennet's negligence, but during Lizzy's appeal, she presents no real evidence. Although Lizzy is more clever than her younger sister, Mr. Bennet could easily see Lizzy's worry as something comparable to Kitty and Lydia's arguing over a hat and suppose that what Lizzy needed most was reassurance.