cyber cheating

My partner & I have been together for 5 years. Recently I found FB messages to a woman he was re-acquainted with at a reunion. I read messages over a 2 week period by which time it turned into cyber sex. I felt that she wasn't as keen as him (she's married with kids) but they did talk about meeting up again in the near future (as she lives in another state). I confronted him and also sent her an email and told her I knew and told her all the things she didn't know about him. She admitted that they'd spent the night together at the reunion although she says that nothing happened as she was very drunk and sick and preceded to vomit every 15 minutes for the next 8 hours. She said that it had gotten out of hand & had filled a void for her & that it was something she needed to address. I was so hurt, more because of the intimacy they shared and the hurt I felt that he could do it to me. We have an amazing sex life although our relationship has had its up and downs. I'm a single mother of 3 and he has never married or had children. I broke it off with him but now we are talking again and I miss him so much. He seems terribly sad and sorry and says he doesn't understand why he did it and that it didn't fill a hole or anything and that our relationship wasn't lacking in anything. What do I do? Do I take him back and believe that he is sorry and won't make the same mistake again or do I get out now? I'm not sure I will ever be able to trust him and whats the point in a relationship with someone who you assume is getting up to no good when he's not with you. Why do men need other woman? Help!