It happens after a person has masterbated. Mainly happens with guys. The person goes to sleep and wakes up late. Typically if your going to take a nap after masterbating you wont wake up for several hours. They will fall asleep and sleep longer than intended. But of course its a great sleep. Thus the Masterbation Coma.
Steve: Hey, Whats up?
Steve: Wheres Bob?
Jon: Ehh hes in a Masterbation Coma right now, Lets go with out him.