When one of your friends gets a girlfriend and rejects everyone else in his life. The girlfriend becomes priority numero uno, and nobody else matters. Disappearing Hangout Guys stop going to the gym, abandon their normal social circles and, in extreme cases, might even quit their jobs.
Mark: Hey man let's get some of the guys together to go fishing this weekend. Call Daniel and see if he wants to go.
Mike: I haven't talked to Daniel in months, ever since he got that girlfriend. Ever since he started dating that chick he's become a Disappearing Hangout Guy.