I might be best friends with your girlfriend, but I know you want to fuck me. She's gone for a few hours, and we're left at the house alone. You try to resist my sexual advances, but I know you can't say "no". I let you watch me masturbate until you can't take it any longer and start stroking yourself. The inevitable happens, and we end up fucking behind her back. I know you'll think of me whenever you fuck your girlfriend, my best friend, after this. ;)