I believe that it is just fair to resign if you realize you have lost. I handle it like this and believe most should:
If I lose, but the enemy is likely to finish the game within 1 or 2 minutes, I'll give him the satisfaction of ending the game and will just play quickly.
If I lose, but it will take some time (let's say you have 7 curses, while the enemy is done trashing, but there is no extra buy and the game will at least go another 8 rounds) I'll type "gg" and resign and usually am up for the challenge of a rematch.
If I can win quickly, I'll end as fast as possible to not make the enemy suffer through the loss. I mean those situations, where you are certain to win (let's say we had curses and another stack is bought out and I have 16 and instead of getting 2 provinces, I'll just buy out the third stack to sack the win).
If I didn't end this way, I'd totally understand if my enemy resigns and doesn't want a rematch because I'd consider making him suffer as unmannered. The same goes vice versa.