We recently became friendly with a chap through a mutual interest. We have become firm friends and he often comes with us for our days out. He has a partner, whom he lives with, but we have yet to meet her, as she doesn't have any interest in our hobby. It is quite an exclusive thing so I don't want to say what it is as I want this to remain anonymous. We have been friends with Harry for about 6 months and I have discovered that my wife is secretly seeing Harry for sex. She has never been with another man except me but has always been adventurous and we have discussed in the past that she might want to try another man. She has always said that she isn't interested as we are very close. and we do have a very good sex life. I discovered it when Harry visited us one day for coffee and I was sent to the shops for some milk. It is about a 10 minute walk to the shops so with shopping it is probably about 25 minutes for a return journey. I had just got to the end of the road and discovered I had left my wallet in my other jeans so headed back to the house. I didn't have my front door key so went around the back and into the kitchen. They were still in the lounge and I could hear the unmistakable sound of my wife having an orgasm. I quietly crept to the lounge door and looked through the gap by the hinges. She was on her knees leaning over the sofa with Harry behind her thrusting into her. His trousers were by his ankles and she had her skirt pulled up to her waist with her knickers on the floor. He continued thrusting into her and after a couple of minutes she again orgasmed loudly. Harry kept fucking her and he came in her about two minutes later with about 5 or 6 loud moans as my wife again moaned and said she could feel his spunk go in her. I was jealous as hell, but also it had turned me on so much and the feelings were so intense, Jealousy mixed with lust.... I quickly backed away and quietly closed the door and waited for a couple of minutes before I entered the door again saying loudly I'd got most of the way and realised that I'd forgotten my wallet. My wife came into the kitchen and said that it's not worth bothering now and we'll have black coffee. They both looked the picture of innocence as we chatted and eventually Harry left. I told my wife that I felt quite horny but she said that she wasn't in the mood but I said you were in the mood 30 minutes ago. She went red and said what do you mean and I explained that I had watched Harry cum in her. She started crying but I said I was ok with it as I loved her and knew it was just sex. She then proceeded to tell me how it had started when I was working away for a couple of weeks and Harry had come to the house and she was feeling horny and one thing led to another. I am not surprised as she is very attractive and I had often seen Harry staring at her boobs as she often goes without a bra and has huge nipples that stick out. I told her I was ok with it and I would like her to tell me about the times she had been with him. She said we had better go to bed and I experienced what it was like to put my cock in the slimiest, warm pussy I had ever had. She said he always cums a huge amount and it was running out of her. She wanted to feel my cock after it was covered in his cum and rubbed it hard as she got it all over her hands. I then came in her and she came again. They had been having sex since about two weeks after we had met him and usually about once a week. I hadn't noticed any difference in our own sex life in that time so we decided that she can carry on using him for sex. She doesn't want him to know that I know, and said that now I know she wants to have sex with him a couple of times a week. He has even stayed at our house overnight and she went to join him in our bed once I 'was asleep'. She seems to be continually horny at the moment as we both enjoy her illicit affair.