My favourite is a CD of Bach's Cello suites - very soothing and uninstrusive.
Hopefully played on viola, a vast improvement!! Though if you're having a 2-girl session try the B Minor violin partita, as each movement has a Double ...
... though Benjamin Britten's "Simple Symphony" can't be bettered for punting:-
I. Boisterous Bourrée
II. Playful Pizzicato
III. Sentimental Sarabande
IV. Frolicsome Finale
That's got to be an hour's worth! ;-)
Seriously, of the four WGs I've seen twice each (not including the rip-off Romanian), two had pop music on during the sessions - but I can honestly say I've completely switched off from it so much that I doubt it was even on at all, but it must have been. Just shows how your mind can play tricks on you, I'm so aware of noise that I sleep with a fan on next to my head, but sometimes after I get up I'm not "aware" the fan is still on for 30 mins or so.
I think Ravel's Bolero is the archetypal music to have sex to ... ?