Props to you, High Treason. I'm a guy and I found out through trial and error that I'm very much the same way when it comes to physical intimacy... i.e. Sex with someone I am close with, care about, involved with, or love: Awesome.
Sex with someone who's little more than a warm body: Not so awesome.
Sigh.

I do like Hamster's recommendation, though.

Has your girlfriend specifically explained why she no longer feels passion in that way?