I don’t think there is anything immoral even if you want to cause pain though, as long as your intent is not to hurt a helpless or non consenting individual. There are obviously safety concerns and being smart. If you think you are going to get so out of control where you may hurt someone intentionally or recklessly, then ya, don’t play. Although I personally would not play boxing because of the greater chance of a head injury compared with other sports, I don’t think there is anything immoral with it and it can be an enjoyable sport to watch. But beside the head, I’m all for contact sports.
And come on, causing bruises or smacking consenting people around in sports for fun is in no way sinful. In fact, the bruises and battle wounds you get are generally something to be proud of. Again, just don’t be stupid–bruises and the like are fine–breaking bones and causing injuries requiring hospitalization–obviously not ok if done intentionally. Why is this topic even up for debate?