I appreciate your thoughtful response, but I feel it is flawed for two reasons. First, your examples deal with bounds of privacy between strangers, whereas by entering into a game you accept some minimal set of mutual expectations. What those expectations are we are trying to decide, but it's nothing like my telling you your girlfriend is ugly.
While my salary level is a question of privacy, the attractiveness of my girlfriend is clearly not a private matter (I mean, she doesn't wear a veil), and neither is my eating meat. They're just areas of my life that other people don't get to impinge upon without me inviting them to, and where there's no expectation for me to invite them to answer. There are a LOT of such areas, those were just a couple of random examples.
The point is, the feelings of other people do not have primacy in terms of courtesy.
In terms of resigning, we're balancing two things:
1. The feelings of the losing player who, apparently, does not want to wait for the game to play out.
2. The feelings of the winning player who sometimes wants to see his engine play out.
The reason why #1 takes primacy in this circumstances are a few-fold:
The losing player is already dealing with the disappointment of losing, while the winning player is experiencing the satisfaction of having won. Asking the player already more disappointed to deal with another minor irritant for the benefit of a player who is already winning seems ungracious at best.
The joy of seeing one's engine come together just seems like it has less value than the irritation of having to sit attentively and push the buttons for a game that is a foregone conclusion.
In general, the broad social rule is that players do not need to play games that they are not enjoying, unless there is a compelling reason why they should. In the same way that it would not be polite for me to insist that my friends play Dominion with me even if they don't like it (just because I do like it), it is not polite to insist that someone else continue playing a game that they have lost just for your satisfaction. The exceptions to this are when a player leaving the game has material consequences on the game, rather than just another player enjoying it.
I have admitted elsewhere that demanding the other player keep playing would itself be rude, but requesting that they wait briefly would probably not be in most circumstances. Regardless, you cannot unilaterally resign without regard for the expectations of your opponent and still be considered courteous.
Sure you can. The expectations of that opponent are unreasonable. There are plenty of things that my opponent might expect, but where I do not have any obligation to respect their expectations. As someone's signature on this board notes, "Mountebanks are for jerks." Some players really dislike cursing attacks. Others may not appreciate my looking at a board with an interesting but slow engine, and opting instead to Doublejack the game into a quick, largely brainless finish. Others may expect me to greet them with full sentences and come up with something original to say upon the finish of the game.
Those are all expectations that people have, and they relate to their feelings, but the truth is that courtesy deals with a small number of formal obligations. "Being a super-great person" may involve a lot more consideration towards your opponent, but
courtesy does not.