Quoting Stacey30:" We opened our relationship up about a year ago and he allowed me to see other men. Everything was fine ... [snip!] ... in contact with that guy. It's a f'd up situation, but one I feel he brought upon himself too since he agreed I could stray."
What he did definately crossed the line. You need to talk to him about what happened and let him know that is wasn't fun or funny and that he needs to deal with his anger about the situation.
The fact that you two tried having an open relationship and it didn't work (never does!) is no excuse for him to let out angry feelings on you with physical agression. I think you should do some couples councelling.
I want to be clear about this - the fact that the "open relationship" idea didn't work out and had unintended consequences (feelings developed) DOES NOT give him an excuse to be abusive!!! If he is really unhappy he can walk out the door, or he can give an ultimatum, or he can talk to you like an adult, or he can suggest councelling, but he CAN NOT put his hands on you with anger!