I suspect this is not the answer, but it fits so well...
Bob became a Mile High Club member, meaning he had sexual intercourse or arousal on a plane.
Bob has never been a M before.
Shortly after takeoff, he becomes a M. Nobody else on the plane is a M.
This means that Bob is experiencing sexual arouas from the planes vibration, and not having intercourse with another passanger (no, I'm not making this up, this is on the Wikipedia page of Mile High Club).
When he arrives at his destination, he's no longer a M.
I think because it was only an arousal, the title is taken away from him as the flight is over.
He boards another plane to go back home. When he arrives, he's a M again!
On his way back, he actually has an intercourse and will be a member of the Mile High Club forever!