While emotionally appealing, the problem with that type of "chivalry" is that you may get to enjoy the sights and sounds of your local county jail. Unless there's a couple of very helpful witnesses to the catalyst behavior, you're going to get slapped with an assault charge, moreso if it was verbal sexual harassment you started a fight over, or just so the original assailant can save face regarding the situation.

I'm not advising that you never come to someone's aid, but be careful before you commit a misdemeanor crime based on someone's claim, especially when alcohol may be involved.