Crying is just another way for emotional release from within the body. I think it’s completely normal to cry after and during sex. If you (or your partner) cries, it’s usually a good sign. It means you’re feeling all sorts of emotions going on and it’s too much for your body, so you cry to let out some of the excess emotion.
Just think about when you cry because you’re overly angry, upset, sad, whatever. It’s the same principle. Remember that last time you felt so strongly about something that you cried over it? Or remember when things didn’t go your way and you cried too? It’s just an overwhelming rush of emotions that causes your body to go into this “release” channel, thus you cry.
This is my own theory. I can’t provide concrete facts, but it’s just how I like to think of it.