Violence is perfectly reasonable in this situation if you make it clear to him that if he doesn't back the fuck off, that the violence is coming. It's the 'fair warning' clause of the man code.
Ah... no. Even if you believe in all this "man code" crap, it still doesn't work.
The only reason why anyone in this situation would seriously think about beating the guy up was that if they thought that guy was a threat to them. Meaning: If Guy B kissed Guy A's GF, Guy A would only think of beating that guy up if he thought B might successfully lure her away from him. Thoughts of violence are a sign of insecurity - that the other guy just might be a better man. That's the only reason that anyone would want to beat the guy up. It shows there's a lack of trust in the girl.
Turning away from violence - *not* wanting to beat the guy up - is the only sign of security. It shows that the guy is NOT a threat, that Guy A trusts his girlfriend enough that she'll know that B is just a jerk, and that she won't abandon A for B. Threatening to beat B up is a sign that A isn't confident in himself or the girl and that he doesn't trust her.