I personally think there is nothing wrong with fighting for sport (i.e. sparring) if both combatants agree on set terms, in a suitable setting ( a gym or karate class/arena) with proper gear if applicable. Now when I say fighting for sport, I am talking about martial arts, and hand to hand. Basically I am saying it is ok as long as it is just training, and both or all involved or willing.
When I spar with friends, both of us go all out. Not to say we are trying to injure one another, obviously, but we won't pull punches, as it helps us should we get caught were we would have no choice, with your life on the line you can't afford to hold back.
Fighting should always have a cause. Normally that amounts to only fighting in self defense.
Now I am by no means muscular, I am probably a beanpole, but I have had a bit of the basics. In other words, I really have to rely on technique if I were to fight. Fortunately I have never had to use it, and hopefully never will, but I enjoy combat. I like kickboxing, MMA, took a beginner's course in Brazilian judo, and I participate in S.C.A. light combat. I have my own suit of full body leather, with a leather brigadine chest piece, although my 18 gage spartan gladiator helm is too thin, I need 14 gage steel. But like I said, although I enjoy combat, it has to be with a purpose or cause, that cause can be for fun, recreation, but it has to be the sporting kind. In other words, I will never pick or start a fight. I won't fight unless it is necessary.
Now I pose a question. Do you believe it is alright to step in to help someone who is being attacked, and obviously in danger and doesn't know how to defend themselves?