Cassel and the offense scored enough points to win; anytime you place 31 points on the board you're supposed to win. Cassel and offense, like the defense, folded when it mattered most.
It is a team sport, you win as a team and you lose as a team. The Texans defense was almost as pathetic as ours, but it is no less a win for them as it is for the Texans offense.