So, when I was 20(M) with my 21(f) girlfriend, we frequently visited the library to study. However, as with most college students, we would end up fouling around, especially when there was no one left. Well, one such night, my girlfriend had her feel of studying and made it know that she wanted cock. So she slowly started jerking me off as she used to, however, today she wanted more. She quickly replaced her hand with her mouth. It was around this moment, that I realized we weren’t alone. There was some other late night studier who was enjoying the view. I pulled up my girlfriends head to show her we had company. This only excited her more. She than got on top of me, reverse cowgirl, and singled for our voyeur friend to join us. We he got to us, my girlfriend made it clear he was only there to lend a mouth. She brought his mouth to her clit, where he proceeded to move back from her clit, to my balls and back to her clit. Between my cock and his mouth, she reached her orgasm rather quickly. However, she wanted to watch me finish. She dismounted me, and had our friend start bobbing up and down on my rock hard shaft. Given his oral skills and my girlfriend working my balls, I pulled our friend down on my cock, as far has he could as I lined his throat with cum.
He quickly wiped his mouth waved at the both of us and left. We would never see him again, but thought of that night for several years afterwards.
Source: reddit.com/r/eroticliterature/comments/smex32/handjob_leads_to_bi_threesome