There have been a couple times I've found myself excitedly getting into bed with gorgeous women... and suddenly being turned off by a certain lack of emotion on her part. In one case I didn't know what it was and she took it personally and started crying, which somehow put me back in the mood because she was being open with me. (I felt like a pervert because of it but I got over that pretty quick
) The other time it was a girl I'd been friends with for years and we were drunk on New Year's Eve. We started making out, and there was a shift in her mood, like I'd gone from being a friend to a one-night partner. I told her I needed more connection or emotion or something, I forget... (like I said, drunk. I'm pretty straight and ended up making out with a guy to turn her on.) ...and she tuned in and it was all good.
I love sex, even pretty casual sex, but there has to be some sort of emotional openness or I can't get it up. She has to want to be there in body, mind and heart, even if it's only for one night.