I posted a version of this story in a comment but wanted to elaborate more in a full post about the time I (21M at the time) shared my girlfriend in college (22F at that time). DM's welcome if anyone wants to hear more details.

To paint a better picture, I'm a tall and fairly athletic (but also pretty nerdy) guy with short hair, and she was a short, thin and petite blonde with long hair and a killer body (she was a yoga teacher by profession).

Well it all started after her and I had a few drinks together and started talking about kinks. Finally i shared a fantasy I had been thinking about all the time- sharing her with another guy. She was hesitant when I first brought it up, mostly thinking I wasn't serious or that I'd regret it if we actually tried it. But she enjoyed how much it turned me on, so she started talking to the kink during sex. Soon she found that it turned her on quite a bit as well, and she actually suggested the idea that we try it if the right situation comes up. I was obviously enthusiastic about the idea but didn't know if that perfect situation would ever materialize.

Turns out I didn't have to wait to long. About 6 months after we first had the conversation, her and I were having a movie night in with my roommate that consisted heavily of alcohol. After we were all several drinks in, I left to use the restroom and I was surprised when I got a text from her from the other room saying she could tell my roommate was checking her out. She said it was kinda turning her on how obvious he was. She then asked many times to ensure I was into the idea if something happened- and I considered to reassure that just the idea was turning me on. After she was convinced, she asked me what she should do to make it happen. I suggested she throw on something sexy.

She went to our room and changed into just a thin, fairly short satin robe. It was thin enough that the form of her perky tits could clearly be seen through the material- and the implication that she was wearing nothing underneath was quite clear. Both of us then walked back out to the main room to continue the movie with my roommate.

After she came out in the robe, what little capability he had of hiding his attraction to her was gone. I immediately noticed him eye fucking her, but pretended not to notice while it made me hard as a rock. Finally I wanted to encourage him to make a move, and so I decided to give him a great chance by excusing myself to bed while telling her she could stay up and finish the movie if she wanted to. I then went off to our room, and listened against the door in anticipation. It was at that moment I realized we had an internal security camera and that if I logged in, I could probably see what was going on.

As soon as I pulled up the feed I could see the chemistry. They were both sitting next to one another on the carpet, almost touching and barely watching the movie. I didn't have audio but I could tell by their laughing and eye contact they were flirting heavily. Suddenly I see him lean in and whisper something in her ear. She tells me later that he leaned in and asked her straight up if he could see her without the robe.

I watched as she moved on the carpet in front of him on her knees, made eye contact and opened the robe while facing him. Without hesitation he lunged in and started kissing her and grabbing her tits. They made out for a few minutes before she unzipped his jeans and started sucking his cock. After about a minute or so, he stood up suddenly, grabbed her hand and walked her into his bedroom. Unfortunately where I lost the ability to watch everything unfold.

She gave me the details. The second she was in his room, she fully stripped off the robe and he threw her onto his bed. He immediately started eating her out. Even a few rooms away I could hear her moans. He continued until she was close and then stopped, and without hesitation put her legs up on his shoulders and slid inside her raw. He fucked her hard, putting his hand over her mouth to stifle her moans. It was at this moment I realized he had no idea I was okay with any of this- he didn't care. He just wanted to fuck her (I can't blame him there). To this day, the roommate doesn't know I was aware they were fucking and into it.

He kept going fast and hard until he unloaded inside of her- hot knowing she was on birth control but he didn't know that at the time. He was willing to risk it. They fucked 3 times that night before she came back to my room- she was soaking wet and still incredibly horny. We tucked multiple times that night as well with how much of a turn on this all was.

Long story short, this wasn't the only time they hooked up. The next few weeks before his graduation, my roommate continued to "sneak around" with her- mostly entailing coming into our bedroom in the morning while I was in the shower for a quicky or fucking her around the house when I worked late. Little did he know she gave me all the details as soon as I got home.

Me and this college girl ultimately broke up a few years later- simply from growing apart as work drove us to long distance. This series of events is still one of the hottest experiences I've had in my life. I recently got to experience sharing my wife for the first time (a petite brunette) which might be the only thing that tops it. Story to come if anyone is interested in hearing it.