Does watching TV series that contain women without feeling guilt, or looking at them without lust, mean making permissible what God has forbidden, given that the religious ruling was not known at that time?

Not feeling guilty when committing a haram act does not mean one considers it permissible. الاستحلال (considering something permissible) is the belief in its permissibility and the denial of its prohibition. And whoever believes that what Allah has forbidden is permissible has committed disbelief (kufr), as Ibn Taymiyyah mentioned.