In theory couldn't someone say I was a bad sport if I resign a game after losing a rook or queen if I blundered it? I wouldn't want to seem like a quitter.
Why would that be "a bad sport"? A bad sport would be to just continue a clearly lost game. Resigning is the proper thing to do.
An interesting question is: If you are clearly in a losing position, with significant material down, and you continue the game nevertheless because you have much more time on your clock than your opponent, would that be being a bad sport, opportunistic, or not? You are essentially trying to "steal" a victory from your opponent by abusing the clock rather than playing the game. I suppose two arguments could be made:
1) If you are in a clearly losing position, with no logical possibility of winning or even drawing against an opponent of their apparent strength, the sportsmanlike and gentlemanly thing to do is to concede, to essentially indicate "yes, I have been bested, this is a clean victory for you, you were better than me in this game". Trying to abuse the clock to get a victory by time in this kind of situation is opportunistic, low, disrespectful and a dickish thing to do.
2) If your opponent is indeed low on time, while you have plenty of time, that means your opponent used a lot more time than you in order to get into this winning position. He will have to prove that he can convert it into an actual victory using the time both of you have been allotted. The clock is a resource like anything else in the game. If you use up your resource and end up in a precarious situation because of that, that's on you, not on your opponent. Your opponent used his time more smartly. If you used up all your time to get into a winning position, you have to demonstrate that you can convert, using the time you have. If you can't, then you were not actually playing better.
That being said, regardless of which of those arguments is "correct", I have played many games where I felt that my opponent actually resigned way too easily. Such as for example me getting a rook in exchange for a bishop and a pawn, with tons of pawns and minor pieces still left on the board, and the situation not being clear at all. Many people at my level (ie. not very strong) will very quickly resign even from a relatively minor setback like that, even though they might still have a chance for a comeback. I think they have been instilled the notion that "rooks are better than bishops" and will immediately give up if they experience that exchange (even if they have a pawn or two more than their opponent).
Good sport and this site doesn't match