A girl who activeley pursues a man with a gf...

is seriously the most pathetic person in the world. I found out three days ago that my boyfriend of 5 months lied to me. He had slept with this girl that I didn't even know he knew. Apparently they had been sleeping together before we met and then two times when we were going through rough patches and I told him I was leaving him and then we worked it out- but he never told me about it- a friend finally did. I was sickened. I confronted him. At first he lied and said he didn't know what I was talking about. He then admitted to it having happened once on one of the nights I told him to leave. He said he had nowhere to go and she kept pushing for him to come over. The second time happened about the same way. I never knew about any of this until Sunday. When he stayed elsewhere, he always told me it was at a buddy's. Anyway, I finally demanded to see the messages between them and he refused for the longest time until I said that was the only way I would EVER consider trying to work this out. He decided to tell me everything. He showed me the messages. They had been texting back and forth nearly every day for months. Now granted, she was always begging him to come over- he was always saying no. She would say she missed him, he would say nothing. There were even parts where he would tell her how much he cared for me and that they would never be together. Still she continued to push and try to guilt him, or be in his head when we were having problems. He would mostly ignore her. But when she texted conversationally, he would answer and when she seemed desperate he would try to calm her down. She even brought him medicine to work one day when I COULD HAVE done it. And of course, the two nights I kicked him out for coming home completely wasted and trying to pick fights, he replied to her messages and went to her house. He said he barely remembered the sex and felt awful and guilty in the morning. He said he told her both times it was a mistake and then came and begged me to take him back after our fight (and I did, not knowing about this girl). When I read the messages and heard the story I felt completely betrayed, probably more so by the messages than the sex.
He's apologized so much. I am trying to forgive him, more for me than for him. He grabbed the phone, texted her "I love Christine, I told her everything. Don't contact me anymore" and when she wrote back freaking out he ignored it. It feels sincere. I've been cheated on before and I won't be with someone I think would be with someone behind my back- but does this qualify? I told him to leave and told him we were over those nights- bad on me. I have issues when the going gets tough. The messages really bother me though. Even though he never said anything that was remotely suggestive of him liking her or leading her on, the mere act of messaging back and keeping the secret with her betrays me, don't you think? And as I said before, there is no one as pathetic as a girl trying to steal a BF. If they wanted you, they would be with you. Ladies, we are classier than that!! Sorry so heavy for a Tuesday morning, but opinions please