Lack of anonymity, is according to this one random person with loads of "good old times" experience and plenty of insights in the current "20 to 25 years old scene", the most important reason.

Where up to 10 years ago one could meet people and exchange kisses without even considering exchanging phone numbers, that is now not even an option anymore.

With the exchange of telno/FB/Insta, instantly there is no more "casual" relation, you are traceable and identifiable. One nightstands turn in to creepy ghosting or stalking really fast.

EDIT: completion of a sentence.