My girlfriend, of six weeks, Emma took me to a party at one of her friends houses. I had met Laurie just once before and thought there was something a little strange about her but put it down to meeting someone new. She definitely looked sexy enough with her short skirt and tight blouse and no bra. The party was soon in full swing and I met a group of blokes and we were talking cars as we propped up the bar. It must have been well over an hour since I last saw Emma, and thought I had better go find her or she would think I was neglecting her. I wandered around down stairs and in the large garden but couldn't find her. Back in doors I noticed that a few of the blokes I had been talking to were also missing. I needed a pee so wandered upstairs to the toilet. There was someone waiting so I joined a queue. As I stood waiting I heard moans and groans coming from one of the bedrooms. I was intrigued as to who was getting fucked and my curiosity was greater than my need to pee. I wandered down to the door and slowly eased it open. There was four men and two women in the room. The two women were in a sixty nine position and I could see that the one on top was Laurie, but couldn't see who was underneath. As well as them licking each others pussies there was two of the men fucking them. My thoughts about Laurie were confirmed, firstly she was a little strange performing in an orgy and secondly she was a sexy thing. Also she did have a sexy body. The guy fucking the girl underneath grunted and moaned as he dumped his cum in the girl. He was quickly replaced by one of the two that were waiting. The guy that had just come zipped up and then left the room. The guy who was fucking Emma was getting ready to blow and the other spare man offered me the next go. I unzipped and got ready for my turn. I was already quite hard from watching. Then the man came and moved away. I was in there and sliding my cock into the sexy Laurie's pussy. I had been plowing her pussy for a few minutes when Laurie raised her head from the pussy she was licking. Then I saw it. The dolphin tattoo on the stomach just above her pussy of the girl underneath. I knew the owner of the tattoo. It was my girlfriend Emma. My girlfriend was sucking another girls pussy and had taken two different cocks in her pussy so far to my knowledge, and I was fucking her mate. As I plowed into Laurie's pussy two more men entered the room and had their cocks out getting them ready for their turns. My girlfriend was a slut and until now I had no idea. I finished up and came in Laurie's pussy. Then I was zipping up and left without Emma knowing I had been there. I was now in much greater need for that pee that I had come upstairs for and found the toilet free. It is pretty hard having a pee with a semi hard cock as mine was. When finished I went back to the bar. The blokes there wee still talking cars and had moved onto F1 and I was asked what I thought about the season so far. I noticed that the group was constantly changing with men joining and others disappearing. Then I saw the chap that allowed me to go before him tap one of the guys on his shoulder and whispered something in his ear. A few seconds later the guy who was tapped pulled another guy by his sleeve and the two of them wandered out the room and up the stairs. This wasn't just a couple of girls enjoying a few men, it was now abundantly obvious that they were pulling a train and I had been lucky to be one of the coaches. A while later as I got another beer a chap I hadn't spoken to before asked If I had been to one of Laurie's parties before. I told him that I hadn't. He said that she held one every few months. Then he chuckled as he told me that she was one good fuck and that her pussy sucking friend was just as good. That was my girlfriend he was talking about. Then he said had I got my number. I looked confused and he said that there was a book in the hall that you put your name in and whatever number you were against that is the order in which you get to fuck one of them. He went on to explain that if you had a preference you just let the next man to go before you if your waiting for the other one. It was now clear that the chap that let me go before me was waiting to fuck Emma and not Laurie. He took me and showed me the book. I counted up nineteen names in the book. The chap said that I should put my name down if I wanted a bit of arse. I politely declined. Back at the bar I grabbed another beer and wandered out into the garden. I was engaged in conversation with a young lady called Sammie. We covered several topics before she asked had I had my turn yet. I tried to look confused. With the sluts she added. I told her I didn't know what she meant. She told me that Emma and Laurie would fuck any man that put their names in the book. I had seen Sammie arrive accompanied with a male. I inquired about him and she said that her husband always fucked Laurie if he could. I was shocked and surprised. She joked that at least if he shags those sluts at least he's not running around trying to shag any other bit of skirt that would allow him. Then she chuckled and added that it also gave her license to have some other cock herself if she wanted. Then she made it obvious to me that she wanted and it was me that she had her sights on. In a few minutes I was in the bedroom, next to the train, with Sammie and she was stripping her own clothes off. I was doing the same. She kissed me for a bit and was stroking my cock to full power. I was fingering her pussy. "Fuck me. Fuck me now doggy style," she insisted. I was behind her in a shot and feeding my cock into her tight pussy. She was a good fuck and she was slamming herself back onto my cock. She moaned and groaned loudly and I was pretty sure those next door would have heard. Then we came almost together. After we rested and cleaned up we left to go back down stairs. As we passed the other bedroom, the noises of sex were still ringing out. I noticed a card on the door with Emma's name on it. Sammie see me look and said that it meant that only Emma was available now. Laurie had call a halt and had finished for the night. I went with Sammie to get another drink. It was about fifteen minutes later that Emma joined us and asked had Sammie kept me entertained. I said that she had. Shortly afterwards we walked home. Emma was obviously uncomfortable by the way she was walking. Luckily it was just a block away. As we got ready for bed Emma went to the bedroom and changed for bed. That was the first time she had changed but not in front of me. She pushed me away when I went to cuddle and said to just go to sleep. I lay there thinking about what had happened at the party and that Emma is a slut. I was struggling with telling her I know and also what do I do about it. Should I dump her and look for a good girl or do I tolerate and enjoy what she is and and does. I was no better than her in that I had fucked Laurie and then later Sammie. If I was a female they would have called me a slut. Just because I'm a man they would call me lucky. I decided that the sight of my girlfriend getting fucked did excite me as did watching her sucking pussy. I decided that in the morning I would tell Emma I knew all about it and that I enjoyed it and wanted her to enjoy herself. When we woke she still wouldn't let me anywhere near her pussy. I told her I knew why and that I had fucked Laurie and Sammie, but not in retaliation. I asked how many men had she fucked and she said she couldn't tell me as she lost count. She also said that she had fucked every male there at some time or another. She then said that she knew I had fucked Laurie as she saw one of my tattoos as I fucked her. We kissed and cuddled. It was three days before Emma let me near her pussy again. After that night I had many threesomes with Laurie and Emma and I joined the train at plenty of parties. I enjoyed those days. They had to come to an end when we decided to start a family. Laurie also started a family. She has four children and has no idea who the fathers are. I am hoping the when the children fly the nest we can go and play again.