A lot of Buffy music is good Valentine's Day music. You know, like your having your girl over for candlelit stuff on Valentine's Day, and the music is playing in the background.
The one I would recommend for that is I Can't Take My Eyes off You by Melanie Doane. It's the one playing in the background when Willow and Tara float as they dance in Family.
If you can't get lucky with that song, give up.