I have downloaded songs and made a mix of them. Annie Lennox' Sweet Dreams Are Made of This is very porno, in a non-vulgar way. Watched a girl strip to it once, although an amateur she was very good at it, and since then I love that song.
Some Sarah McLachlan songs are good for romance too, quite many of them in fact. If you want something in the background for some dirty hardcore, Nine Inch Nails has a few songs in that direction, like Like an Animal. One girl I know would play Foo Fighters, and it worked. Don't know what album it was.
If you want something calm: Katie Melua, Sarah McLachlan. But if you want something faster but not quite "hardcore" or trashy, I recommend the new kind of more vocal metal, "opera" metal if you will, mostly from Germany and Finland and the rest of our region, but there are also some American bands. I am talking about Within Temptation, Evanescence, Nightwish, QNTAL. You have to listen to them first though to determine if this is something for you.
Otherwise there's also classical music: Mozart's Requiem, if your girl likes it dark and cozy.