Hi George, I am sure that you will get lots of suggestions, but for me
nothing surpasses the sensuous celebration of physical love (brushing
aside all moral considerations) in Monteverdi's The coronation of Poppea.
Strauss has depicted intercourse in instrumental music two times (that I
know of), in the Rosenkavalier overture and in the Arabella prelude to the
third act.  As for the madrigals, what you say is true, but it becomes even
more striking if you broaden your definition somewhat to include other
kinds of Renaissance vocal polyphony, like the Villanelle or Moresche,
which seem to deal almost exclusively in matters sexual (and coprophilic,
which I like rather less).  For my money, nothing can beat the Concerto
Italiano's disk of Orlando di Lasso's rowdy wordly works and songs on Opus
111.  That is a pure delight and definitely worth checking out (I think
this CD was orginally marketed with a sticker that read "All you ever
wanted to know about sex" or similar).  And finally, I suggest you check
out the fellatio scene in Thomas Ades' recent opera 'Powder her face'.

Best, Andreas