Even secular society valued chastity less than 20 years ago.

I remember in the 90s and early 2000s, secular society still saw chastity as a virtue. I remember secular authors telling people not to to be pressured into sex from their peers and not everyone is "doing it".

Abstaining from sex before marriage was still looked as an acceptable and virtuous choice by the non-religious. Chaste couples in romance movies were still very popular. One night stands were seen as immoral by secular society. While cohabitation was not looked down upon, it was expected as a stepping stone towards marriage.

Nowadays there are people who say that a relationship with your girlfriend without sex is "just a friendship". Hook ups and one time flings are just "blowing off steam" or "exploring your sexual preferences". Today, Cohabitation is considered without any expectation for marriage but for economic convenience.

Not having sex before marriage is seen as irresponsible and naive. And marriage is now considered superfluous when starting a family. When did this cultural shift in secular society begin?