Sexual intercourse between a married couple and male stranger.
Typically involves the married couple propositioning the male stranger to do The Double M. Motivations for propositioning the stranger vary from sexual dissatisfaction to mere curiosity. Some married couples habitually engage in The Double M and are considered a sub-category of swingers.
For the sexual intercourse to be classified as The Double M there must be full penetration. If the three participants are fondling each other without penetration it is not considered The Double M.
The purist's definition of The Double M stipulates that the male stranger must be a first-timer (i.e. it must be his first time having intercourse with a married couple). Purists also insist that The Double M only involves heterosexual positions (i.e. the two men do not fondle or penetrate each other in The Double M).
Friend 1: "Hey, man. A weird thing happened to me yesterday. I ended up doing The Double M with that creepy neighbor couple."
Friend 2: "Oh ya? What happened?"
F1: "Since I am new to the neighborhood this was the first time we hung out. We were all playing video games when out of nowhere the wife starts rubbing my neck. Her husband was watching it happen and started smiling. Very creepy stuff. Then they both asked me if I wanted to do The Double M. One thing led to another, and before I knew it I was getting pleasured by the wife while the husband was on the other end of her. I couldn't believe it."
F2: "Wow! The Double M! I always thought that was a made up thing."
F1: "I guess not."