Why can't I believe him when he says he didn't cheat?

In December 06 I found out that my husband had been corresponding with his old girlfriend via e-mail. The e-mails started in early November 2006. I read the e-mails and thought they were pretty platonic but I was furious. I talked to him about it and he sent her an in mid-Dec. 2006 ending his correspondence with her. About a month ago I found out that they also talked on the phone (a total of 5 days during that Nov. & Dec.). Again, I was furious. I asked him if they ever met (she lives in his hometown which is in another state). He said, "no." This week I found out that they did meet for lunch. Now I don't believe a word he is telling me. He said that he didn't tell me because he thought I would leave him. My friends tell me that I am crazy that he didn't cheat because if he had, they would have talked on the phone more than 5 times. I can't stop thinking about it. I feel like he has lied about everything having to do with this woman. I can't get it off my mind. Help!