In literature there have been many great writers who have strayed near—or even crossed—the borders of pornography in recounting tales of goings on among the devout: Chaucer and Boccaccio (and Balzac who imitated the latter's style in Les Contes Drolatiques) are in an ancient tradition of story-telling of this kind. In art there are of course many illustrations of misbehaviour in the cloisters such as the erotic watercolours of a certain Viennese painter (I give no link to these: OMF is a family blog).
But in opera, improprieties committed by members of religious orders are seldom explicit: nun/monk love arias are rare and never sung in flagrante delicto. No doubt someone will remind me of exceptions to this, but I can think of only one example of a naughty musical nun, and even then she is probably only thinking about it, or trying not to. In an operetta called Casanova, written by Ralph Benatzky in 1928 to music by Johann Strauss II, there is a nun's chorus; the operetta is hardly ever heard, but a few years ago a recording of this chorus made in 1932 became hugely popular and after a period of deletion had to be restored to the HMV catalogue owing to public demand.
We first hear the nuns at their devotions; the music melts into a waltz rhythm, and presently a single nun (the one Casanova is after?) begins a seductive, swaying tune. As the others join in, she soars higher and higher in voluptuous ecstasy (though the words indicate that she is praying to the Virgin Mary) while a solemn bell tolls, until finally a prim chord on the organ reminds us that we are still in a holy place.
There is a later recording of it here by the Viennese soprano Hilde Gueden; others, including Gracie Fields and Joan Sutherland, have recorded this delicious piece of bad taste with varying degrees of reverence and sexiness.