The first guy who allegedly said that was a "robbery knight" (Götz von Berlichingen) to a collector of his castle, and he said "er kann mich im Arsche lecken!" Mozart and Goethe (1) just took that...well because their where Mozart an Goethe :)
Right. This tension between developing Enlightenment values, which were becoming more liberal among Enlightenment cognoscenti, Beaumarchais, Goethe, Rousseau, Diderot, et al, and traditional values is quite evident at that time in writings and plays of the period. Ideas coming forth from these Enlightenment intellectuals spread like wildfire across Europe and elsewhere despite what today we'd consider a primitive and slow communications network.
For instance, Pierre Beaumarchais' 1784 play, 'La folle journée, ou le Mariage de Figaro', was not only considered frivolous, almost salacious, by much of Viennese society at the time but also controversial by the fact that in 1786 only two years after Beaumarchais' play was first performed in France, one of their own, Mozart, had not only adopted the play into an operatic version, 'Le nozze di Figaro' (The marriage of Figaro) but also managed to have it staged.
Even today, we wouldn't expect readoption of a work to happen with such great speed. In an age when things and ideas usually moved at a much slower pace, it's little wonder Mozart was at the center of controversy.
(In my opinion, it's not only Enlightenment ideas and values that are interesting but also the enormous speed that they spread across both countries and languages. This makes the 20 or so years before and after the French Revolution so interesting from an historical perspective.)
1: https://en.wikipedia.org/wiki/G%C3%B6tz_von_Berlichingen_(Go...