“That’s not really my point, though, it’s more that it never appears to occur to Lydia at any point that putting herself into the power of a man and sleeping with him while unmarried is either wrong or catastrophically dangerous in the context of her time.”
Lydia was like this because her parents had failed her. Mrs Bennet indulged her and shared Lydia's superficial love for clothes, officers, and gossip, serving as a poor moral guide.
Mrs. Bennet also allowed Lydia to "come out" into public society and mingle with militia officers at an unusually young age (just 15).
Mr Bennet completely neglected the moral and intellectual training of his younger daughters, preferring to mock their silliness rather than correct it.
Lydia possessed high "animal spirits" and viewed life as a game, famously calling her elopement with George Wickham a "good joke" rather than recognising the social ruin it brought her family.
Because no one ever disciplined Lydia or taught her propriety, she genuinely did not care about societal rules or the feelings of others.