Each Time We Had Sex, My boyfriend Wrote Down

I was at my boyfriend's house, making up the bed and changing sheets when I saw a diary. I decided to have a peek. My boyfriend wrote down every time we had sex, in detail. He wrote what I did and even what I did not do. I was surprised.
I found out that one morning after he dropped me to work, he was on his day off, he took a girl to the house and had sex with her. This girl was only 17. He is 40 and I am 27. Is this right? I took the diary home and burnt it. He has not said anything to me so I don't know what he is thinking.
I had asked him, before I saw the diary, if he has another girlfriend. He said 'no such thing'. I wrote down the girl's name and her number but haven't called her because I am afraid. I have two children and he helps to support them. Their father does not give me any money. Please tell me what I should do. What you advice to the woman?