I have a playlist, tastefully entitled "Songs to fuck to." devoted solely to this purpose. Naturally, a giant chunk is triphop.
Massive Attack - Paradise Circus, Dissolved Girl, Mezzanine
Zero 7 - Destiny
Doves - Firesuite
Portishead - Wandering Star, Pedestal, Glory Box
Thrice - Digital Sea
Handsome Boy Modelling School - The Truth
Morcheeba - Crimson
Feist - Lovertits
Broken Social Scene - Sweetest Kill, Hotel
Goldfrapp - Strict Machine
The Kills - Getting Down
Jeff Buckley - Grace