There will always be refs or individuals who accept bribes to influence their decisions but it's unrealistic and inaccurate to think the league is fixed. That implies a systematic type of corruption, which I do not think is possible in the era of instant replay and media exposure.
Of all US team sports, I would say basketball is the most likely to be corrupt. It only takes one or two calls to change the outcome of a game and I've seen too many questionable ones over the years.
None of these sports come close to boxing, the most corrupt professional sport on the planet. How many bad decisions will it take before these alphabet soup commissions are investigated? Why are there no uniform rules of judging fights? I think I know the reason and it's pretty sad and inexcusable.
The NFL does a good job officiating games and generally tries to get it right. Every play is under the microscope and there is so much money involved, I like to think a corrupt official would be identified. The accuracy of the gambling "line" on the games (point spread and over/under) is pretty amazing. It's enough to make me consider the possibility of a fix, but I would be shocked if the outcome was not legit.