I've done a similar thing one night when I was in my twenties a buddy and I picked up a local girl that we had known from college. She was pretty ripped and she also used to go out with one of our friends so we sort of knew her a bit.
As the night progressed I ended up fucking her on the lounge room floor, after I was finished I casually asked her if she would also fuck my buddy. For some reason she agreed and went off to his room and he fucked her pretty hard. After they were finished she later came back to the lounge room to gather her stuff.
As she was gathering her stuff I started fooling around with her again and I slipped it in her again. It was a strange feeling, my cum would have been inside her, my buddy's cum would have also been inside her. This for some reason made me get even hornier so I shot another load in her. As I pulled out I remember thinking how strange this was knowing I would have had some of my buddies cum over my cock as well.
After that we sort had a common bond and were also good jerk off buddies as well