When I was young and dating my wife, I failed to stop her one fight because it happened so fast. Started in a flash and was over in less than a minute. I stopped her other near misses before they got going the other dozen times. Now much older and a bit wiser, If I walked in on her fighting, I'd make myself scarce and enjoy the show. If it got too violent or if one didn't stop when it was over, I'd step in, but other than that, I would definitely let them fight.
Also, for Anna and Bob (and anybody else that cares), I served 10 years in the Army National Guard. No thanks needed. I shot M16's, 45's, 9mm, M60, M203 Grenade Launcher, M72 LAW, M163 Vulcan, Trained on Stingers and Chaperal's, Drove HumVee's, Cranes, Duece and Halfs, tossed Grenades, used C4, partied hard and drank way too much. Was some of the best fun I ever had in my life and I got paid to do it. Never deployed anywhere, never did anything. Happy I did it and proud to be an American.