... but not so much of one as to be unable to tell the difference between Lydia and Kitty squabling over a hat and Lizzy's reasoned arguments! He just wants to be able to rationalize doing what he wants to do, anyway.

What makes you think Mr. Bennet doesn't think Lydia is stupid? What has he ever taught her (or what would he believe her mother would have taught her) that would prevent her from acting stupidly? Yes, it's a long way from flirting to something else, but with no meaningful supervision and lots of temptation, what is it that Mr. Bennet thinks will put a brake on Lydia's behavior?

What evidence would you expect Lizzy to present? Lizzy:

1) details Lydia's improprieties to her father

2) argues that friendship with Mrs. Forster will not improve her

3) argues that Lydia will demonstrate even less propriety in Mrs. Forster's company in a place like Brighton than at home

4) argues that Lydia's reputation is a disadvantage to the family and a threat to their respectability, particularly in the case of Jane and Lizzy