We can all agree that it would be wrong to pay to watch two people fight to the death, even if they consented and were paid money for the fight. We can all agree that it is not wrong to pay to watch people play golf, even though a stray golf ball could potentially kill someone. So we can use these points as the end of a spectrum that represents the ethics of watching, and thereby contributing to, a sport.
'Mortal Combat'--------------------------------------------------------'Golf'
Most sports we watch are somewhere in the middle. But, almost all sport has some level of genuine risk. So if we agree that there is a risk level that is too high for consent (i.e. 'lose and you die' risk level) then where do we, as individuals draw the line?
Personally, this is a major reason why I don't enjoy boxing. It's just too brutal for the real live human beings who participate in it. It causes a lot of harm, and as a viewer I can't escape from the images of an addled George Foreman trying to remember a single sentence long enough to film a commercial for his grill. Boxing smashes brains, so I don't feel ok watching it.
As we gain more information about the dangers of football, and as the game seems to become more concussion prone, I think it is worthwhile to really consider where football falls on the spectrum.
Personally, it hasn't fallen below boxing yet, and I still watch football every weekend. But it was just a thought that I think is worth rolling around from time to time.