husband cheating on internet

I recently learned that my husband has been exchanging sexual emails with a woman he went to high school with and recently reconnected with on Facebook. I learned of the emails because the woman involved shared some of the emails with a friend of hers who happens to be friends with the sister of an old friend of mine. The information eventually got back to me. I have been with my husband, who is 10 years older than me, for 15 years (married 5.5 years), since I was 18 years old. I thought we were best friends–we spend most of our time together and get along well. Moreover, while we don’t have sex as often as I think we should, it is amazing when it does occur. I’m shocked, incredibly hurt, and humiliated by this information. He admits only to exchanging “flirty” emails with the woman after she started it and says he had no intention of cheating. I have no idea what to believe, do, or feel. Please help. Thanks. – CyberCheater’s Wife