S&M means not sex & (whatever)!!!! Listen to the song: "The sticks and the stones will break my bone, but chains and whips excite me!" It means BDSM - Bondage - Domination - Sadism - Masochism It is a Sex-game with pain.
S&M stands for sadism & Masochism, sadism means that あなた get pleasure from inflicting pain または によって humiliating someone and Masochism means あなた get pleasure from receiving pain または によって being humiliated, so sadists and masochists are basicaly meant for each other and that relationship is the foundation of bondadge, whiping for pleasure and the dominatrix