How about an indication of whether you want to play using diplomacy or not , which is there in the actual game set up menu?
It could stop a lot of arguments- let's face it there are two opposing points of view about using diplomacy in these games,so why not make it clear how each game is to be treated, even before the games start?
I would be happy to play in either type of game.
I am pro diplomacy personally , but I fully respect the non diplomacy guys' point of view as well so I would play according to the game setup for each type of game.