Each case has to be evaluated on the circumstances, although some insurers (Tif) have a poor reputation. If football wasn't proscribed in the policy as a dangerous activity ( unlike say scuba diving while not qualified) then there's no basis for refusing the claim. If you were injured in a road accident not your fault but weren't wearing a seatbelt,your damages would be reduced,not denied,on the basis of contributory negligence.
Quite a minefield!