It depends on my mood... if I'm up to normal vanilla sex (or if I'm expecting it already) and the girl starts crying, I go easier, since I'm probably hurting her unintendingly.
However, if I'm in the mood for rough sex, the one in which I slap the girl, fuck her hard or generally humiliate her, and she starts crying... I pretty much have to hold myself back not to cum the instant I see the tears and hear the sobs. It's a great feeling.