I am not a lawyer, but I am a CCW permit holder and have had training in this area. As the others have pointed out, the laws vary from state to state. Where you are has a great impact both in the literal application of the law, as well as what actions police and prosecutors are likely to take, and what a jury will decide. You're not going to have the same experience in San Francisco as you would in Hazard, Kentucky.
In general, the case seems to be that an assault that does not involve a weapon may or may not justify deadly force in return. In other words, if someone punches, shoves, or slaps you, you may not even be entitled to use any force back, if you believe the attack has stopped. On the other hand, if a person is unarmed but is assaulting you in a way which you believe your life is in danger, such as the Zimmerman/Martin case, then you can make an argument that lethal force was justified. In between, if a person is physically assaulting you and you can take moves to protect yourself, while seeking help and trying to back away, it will likely be justified in the eyes of the law.
Most states also have laws that cover intervention on behalf of someone you are associated with, in terms of trying to stop an attack. I am not sure about good samaritan laws in this respect - any training I have had has always been to remove yourself from the area and call the police. Now, if the attack is over and you punch someone, then you are on the same legal footing as they are. If someone is repeatedly hitting your wife and has not stopped, then you're going to be on more solid ground if you try to stop the attack - with force proportional to the danger - than if it were over and you got a cheap shot in to make yourself feel better.
Honestly, the best thing to apologize calmly and leave the area. Violence that occurs between two strangers often is because both parties have escalated with threats, shouting, insults, and so on. Additionally, if you are keeping calm and the other person does escalate to physical violence, then it will look far better to a prosecutor or a jury. Just don't fight if you don't have to. Let's say someone punched your wife, and you punch them back, and they hit their head on a curb and died. Well...study up on how to survive in prison.
I practice martial arts, both armed and unarmed, know how to use firearms (and carry one often), and have forgotten more ways to kill someone with my bare hands than most people know, and I avoid confrontation like the plague. I don't even blow my horn when someone cuts me off, and I am polite to everyone. Even if you're justified in self defense, it brings more problems than it's worth if you can avoid it. If you're not a cop, don't act like one.
Also, I am not a lawyer, but the above should be a starting point for helping you get familiar with the laws in your area and what you can and can't do to protect yourself and loved ones.