It was the reality of the time - there were horrendous double standards & JA to my mind deserves credit for her honesty about this rather than romanticising men’s behaviour by tropes such as reforming a rake.
We see these double standards very clearly in Sense & Sensibility where there is an offstage character who is seduced & her life is destroyed. Her seducer is accepted back into society without question (although I think one of his relations tries unsuccessfully to pressure him into marrying the girl).
The word seducer does show where the guilt lies (even if there were no consequences for the men) & there were earlier books such as Clarissa & Les liaisons dangereuses, which explored the motivation behind seductions.
Re Lydia, it’s a lack of modesty & proper shame that she is criticised for all through the book - in her behaviour before the elopement, during the elopement & her lack of any shame when she returns home after being married. She was very lucky that Darcy wanted to marry her sister so was prepared to force Whickham to marry her. I think it is the lack of remorse & shame that is the difference between Lydia & Georgiana.
There’s also the responsibility of the adults looking after both Georgiana & Lydia to consider. Darcy says in his letter something along the lines of being sadly deceived in Georgiana’s governess & Lizzy tries to warn her father that Brighton is not a sensible place for Lydia but is ignored.
We definitely see things very differently now although there are still some areas of behaviour where people have zero tolerance for teens stepping outside of social norms (eg riding electric motorbikes).