As no one else has stepped up and said it, I feel obligated to admit:
I am a crier.
No, it isnt because I hurt in anyway and I doubt that this guy's girl is in any physical pain. No, it doesnt happen all the time...it actually only happens when it's a really, really, REALLY intense orgasm. No, I dont cry when I masterbate, no matter how good it is. And it only seems to happen when I'm with someone I'm extremely comfortable with (never one night stands and such, no matter how great the sex is).
I honestly dont know why I cry. As only really intense orgasms bring me to tears, I suppose its just the magnitude of the release. If I wanted to try to psychoanalyse the situation, I suppose I'd say that I'm a pretty stressed out person. I've had a hell of a lot going on for a very long time, and I sometimes wonder if its all that pent up emotion and stress that Im not able to display (for various reasons) in everyday life simply exploding out of me when I finally relax enough to have mindblowing sex. Mind you thats just a half thought theory on my own crying. Im not even gonna try to make any across the board assumptions.
My advice to the guy dealing with the crier is to enjoy it! Have the same pride in her emotional outburst as you would if she were a screamer, a biter, a moaner, a clawer, a curser, a preacher, etc.
And whatever you do, DON'T freak out on her. I had one boyfriend get weirded out and we had nothing but crappy sex for the rest of our relationship because I could never get really comfortable with him. It made things very boring. Just hold her tight and feel manly; you're so freaking good shes crying with gratitude