As for young people fucking less, it's a lot of reasons really.
More ways to pass time on your own (video games, social media, netflix etc.), much more concern about STDs, less privacy with young adults moving out of their parents houses later and later, dating apps which are used for an ego stroke rather than a way to find partners. I also think younger people are also more insecure and self-conscious due to decline of socialisation (which has gotten wayyyy worse post-covid) and also the chance that their behaviour will be called out/made visible on social media, and in the age of #MeToo there's the worry that men will be branded a creeper when making the first move.
Idk if anyone else has picked up on this, but I've also noticed that something strange is happening where media has been getting more hyper-sexual (Shows like Elite and Euphoria for example) even though youth are more insecure and sexless than they've been in generations. Iirc something like this happened in Japan quite a while ago, where weird sexual expressions worked their way into the mainstream with sexy anime and Japanese businessman porn or w/e, while young men gradually went into their shells and became more and more sexless. Idk if the West can prevent this.