Depends on the reason for the loss. If I was beaten by a superior player, I'll simply tip my hat to him and call it a day. If I lost because of some dirty trick, I'll throw a hissy fit at my controller and play something else to calm down. If I lost because I was absolutely killing it but the rest of my team were a bunch of newbs who suck donkey ass and all killed each other by accidentally throwing C4 at each other for the entire game, I'll sign out of that lobby and search for people with at least a modicum of skill. If I lost because people on my team know full well what they're meant to be doing but were too busy fucking around trying to get some trophy for killing off your team mates or some other crap, it's the one time I'll use my mic to hurl hysterical abuse at other players.
But as a rule I find myself with a brief moment of frustration, then tip my hat when I see the kill cam and know that it wasn't some dirty trick, it was just a good shot, and I move on.