Are only women
doing it? Max Cryer explains:
The genesis of ["everybody's doing it"] can be traced to a Mozart opera of 1790, commissioned by Emperor Joseph II of the Holy Roman Empire and said to be based on a much gossiped about real-life incident in Vienna. Mozart's librettist Lorenzo da Ponte entitled the story Cosí Fan Tutte — which translates as 'Everybody's doing it' (though purists will point out that 'tutte' can be seen as the feminine of 'tutti', and thus only women are 'doing it').
(Who Said That First?: The Curious Origins of Common Words and Phrases