As DM, I cancelled a session due to a date with a hot redhead. One player did not get the message and showed up shortly before the lady...he understood when I kicked him to the curb.
When I was in high school or college, we had one of the guys turn down a date because it would have conflicted with a scheduled game. I can't remember whether he was allowed to play that particular night or not, but it was made fairly clear that anyone who passed up a real date for the game would not be allowed to play the game, anyway. Priorities, man.
Currently, we're in the early stages of building a house -- soliciting quotes, looking for land, meeting with various contractors. I've canceled a game without a second thought because game night was the time that worked best for a meeting. I've also done it because of a program at my daughter's school. I even did it
once when my eldest had an insane amount of homework that was fairly hard and just needed a lot of help (my wife is one of the players, so I couldn't just hand-off to her).
I do not like canceling a game. I especially do not like canceling on short notice. I agree that the GM is pretty much like the host of an event. Even if the game isn't at the GM's house, it can't go on without him.
But, stuff comes up, especially as you get older and have careers, families, etc. If something else is a higher priority to you, then you need to cancel. That cancellation should be handled in a mature and respectful fashion, though, whether you're a player or GM.
If you have enough "higher priorities" that the game (or attendance, for players) becomes erratic and unreliable, then you may have to step up and face that the game isn't a high enough priority and you need to pull the plug, permanently. That's part of handling things maturely and respectfully. Where, exactly, that line is depends upon the group. I've been in groups that demanded a twice-weekly game and would bend their collective schedules to ensure that happened. I've also been in groups that was peachy with a monthly game. Sometimes the only way to determine the collective priority is to have the conversation.
IMO, though, it's not okay for a player or GM to cancel on a game session to go out bar-hopping (other than for a birthday party or some such), a guild raid on WoW, because they want to see a movie, etc. Yes, gaming is just another hobby, and there's nothing wrong with having higher priorities, but it's also a commitment to the rest of the "team" and, generally, something that can be planned for. Either block the game time off as "busy" for purposes of general recreation, or don't. "Tentative" is rude.
