Should I Be Worried That My Husband Arranged to Meet a Woman Without Telling Me?

My husband and I have been together for 15 years. We have long had an agreement that he goes out with his mates once a week and stays over with them. I have always trusted him. Recently he said he was arranging to meet some friends who had contacted him. But I had never heard of them before and for some reason got suspicious. Despite giving him many opportunities to tell me the "truth", I wasn't satisified and resorted to checking his e-mail account. I found a reply to a message from a woman where he was giving his life story (which didn't make mention of me) and ended with him agreeing a date to meet. The date was one where I would be out of town. She subsequently cancelled. I haven't told him I read this e-mail but he did say the people he was meeting were men and when I broke down and confessed my suspicions, he said he loved me, which he hardly ever says. Maybe it is harmless, but then why not mention it?