I just had to answer a question on Quora today. Someone had been hit by a normally gentle and kind boyfriend who "just snapped." She claimed she'd done nothing wrong, but she thought it might have happened because she was studying with a male student. And she made it clear she wanted to be told it's not a problem if it happens just once.
I could not tell her that, so I wrote this, and I wanted to share it with you, too.
It may be just once, but let's get clear about it. Anyone who hits you when you do something wrong is a really bad choice for a partner. Our home should be our safe place from the dangers of the world. Sharing it with someone who feels justified in deciding what's right and wrong for you and making it unsafe whenever he chooses is NOT a healthy life choice.
If he did it because he "just snapped" it means he sometimes cannot control the way he acts. He can feel one way and behave another. This is just as dangerous but often treatable. If he gets it treated, then "just once" isn't a problem. For him, it's a blessing, for you a few awful moments in a long life.
If he does nothing about it, he doesn't care enough about your safety to live with.
How to treat it? That depends on the cause. Drug and alcohol use can cause this. The treatment then is to learn to live without drugs and alcohol. A brain tumor can cause this. The treatment then involves a team of physicians. Alzheimers can cause this, but there's no real treatment yet; you'll need others to protect you. More often, a lack of skills causes it. I like the CompassionPower approach of Dr. Steven Stosny, but there are also plenty of anger management classes available and psychologists who can teach this one-on-one.
The problem with just ignoring it is that no one who really loves you wants to hurt you. This means that every time he hurts you, he'll likely invent an explanation for doing it other than lack of self-control. Usually those explanations make you out to be a bad person and make it less and less likely over time that he'll do anything about his very serious problem.
Your own explanations for it will make you out to be a better person, a person who doesn't hurt the ones you love and who forgives. A relationship in which the two partners look down on each other is quickly drained of love and becomes just a dangerous living arrangement for them and especially for their children.
So my answer is to forgive him after he takes some action other than promises to prevent it from happening again. Or leave him before it can happen again. And make sure you have family or friends around you for a few days just in case he snaps again when you break it off.