How about you corelate this with cost of living, financial independece, and decreasing socialisation? We are seeing similar trends in european countries.
When my generation was growing up, and reaching adulthood, in the late 90s early 2000s while our own teens didn't have places where to get intimate, outdoor sex was way more common than it is today!
Who in their right mind would do so today when you can't even pick your nose without being caught by some random outside camera or mobile phone.
Maybe think radically and enforce the opposit of curfews whereas old parents living with their young adults are forced to hang outside at bars/clubs from 10pm to 9am to give their youth some breathing room :))