I wrote this myself, throwaway for obvious reasons.
This happened last December while Katie(18) and I(20) were visiting my family for Christmas. It started at the mall with Katie and Dylan(22). We were shopping and getting food, so we invited Dylan over to my grandfathers where we were staying. My grandfather had already gone to sleep in the other room. So we went into the garage (he has a studio apartment in a basement). We had just hung out for a while talking and joking with each other. We had decided that Dylan should call someone so we could smoke weed. He called someone as I started making comments about Katie's looks and that she should be wearing less. It got to the point where Katie pulled me to the side in the same room and asked me if I was really okay with it and I said yes so we were all sitting in a circle when I brought up a rule me and Dylan had which started us off a jobuds and eventually a bit more, the rule was that if one of us pulls out part of our dick out the other has to match or pull more out. So she said she liked the idea of that and said she'd do her best to match jokingly as I hadn't officially mentioned what I was getting at yet, so I pulled her from where she was sitting onto my lap and started to feel her up on top of her clothes. I stuck my hand down her pants and rubbed my finger across her pussy as I exhaled down her neck and then I stuck my finger in and unzipped her pants with my other hand. I got up to stand with her and unbuttoned my pants, so she could put her hand down there, as she did Dylan goes "hey what are you guys doing over there" (we were about 2 feet away from each other). So I reached down and pulled the tip of my dick out, so he asked if the rule was in play so I said it was always in play. He then took off his belt and took the tip out and then some (he has a big dick it's 9 inches and thick when hard, it's bigger than mine in every way, mine is only 6.5 inches but it's also pretty thick). I pulled the rest of dick out after that still fingering Katie so she can jerk it off as I go to kiss her I see that he pulled his out and is now starting to jerk off. I tell him to come closer and put her other hand on his and she starts to jerk him off. She then kneels down and takes mine in her mouth, I'm throbbing at this point, he's still about half mast already bigger than me and I could tell she liked it by the way she was jerking him off and staring at it. I then stuck my hand through her hair and started to help her and push her down my shaft to the point she could've licked my balls had she stuck her tongue out and then i grabbed her hair and gently pulled her off mine and looked her in her beautiful eyes. She then puts just the head in her mouth and stairs straight up at me (which is one of the hottest things ever to me). I then guide her head over to his dick which is getting harder and then she starts licking the tip jerking him off, without the slightest hesitation she swallows half his dick while jerking him and I off. He put his arm on my shoulder as she proceeded to suck us both back and forth and at some points our heads together. At this point his friend is pulling up so we have to settle down and roll a blunt. We went outside and smoked, it was really cold so we couldn't wait to get back inside. When we came back inside I pulled my pants down and kissed her long and deep as she grabbed my dick, he follows the rule and pulls his back out and comes up next to her so she can work it with her other hand. As soon as I stopped kissing her she dropped down to her knees and started sucking like she'd never see a dick again. Seeing her look up at me with two dicks starring at me with her gorgeous blue eyes, I almost came there. I moved away so she could focus on sucking his big dick while I took her pants off and slid her thong over so I could lick her pussy. After eating her out for a bit I slid her thong off and slid my dick into her from behind. As I did she muffled a squeal around his dick, I started off slowly thrusting into her until I was slamming into her pussy. I was about to cum so to prevent cumming I slowed down and switched with Dylan. As he is about to slide into her he aske if he should use a condom and I said it was okay if he didn't. When he put his cock into her tight little pussy her face changed with every inch till she was about to scream, he slowly pushed in and out of her until she got used to how much bigger he is. When she was ready she started to suck me off again, he starts to speed up his pace a bit. When he did her mouth started being forced down my dick with each thrust, forcing her to gag at times. He started really pounding her pussy and as she started cumming all over his dick he pulls her down the entire shaft to his balls holding her there as she squirmed and came all over him, he kept fucking her until she came again and as she squirmed on his dick without control she looked up at me with the most satisfied happy glowing eyes, I couldn't help but cum and as I did she swallowed every drop and looked up at me moaning and hugging me while he kept fucking her until he pulled out and came all over her perfect ass. Source: reddit.com/r/sexystories/comments/3h9onz/i_shared_my_girlfriend_with_my_best_friend_mfm