A Munch is a social gathering for people who are involved in, or interested in BDSM.
Munches are casual events, and usually take place in public places like Nando's or Starbucks. This works out fine, because munches aren't for BDSM practice and play. They are a safe place for people to meet others with similar interests, and become more informed about, and comfortable with BDSM.
Maybe if Christian Grey had just gone to a munch, instead of choosing a random 21 year old virgin, everything would have worked out fine.