[MF] (18yrs) Dropped my girlfriend off before prom party to fuck my best girl friend at the party..

This is my first ever experience I have shared so please bear with me if it is not too captivating. The sexual parts are towards the bottom but I tried to have you guys have the clearest picture about who and what is happening and the context.

Alright to paint the setting, it was the first week of May 2016 and my senior prom was finally coming up. To keep up privacy, my girlfriend in the time frame of this (and current one, lol) will be named S and my best friend will be named M (she’s a girl, thought i would point that out).