Millennial hookup culture has always been more rumor than reality, public perception is distorted by the status attached to casual sex (especially for men), while surveys show most people are not that promiscuous and the people having the most sex are married couples
flirting depends on being able to both express desire and accept rejection when desire itself is condemned you get shame and creepy / asexual behavior, when rejection is not ok you get all sorts of pushy behavior and emotional reactivity that make flirting feel unsafe