I'm sure this will sound sexist as well, but the chivalrous guys far outnumber the jerks. If someone crosses the line, by all means call the cops but also consider making an immediate and loud protest, and you might get help from an ad hoc group of temporary big (or little) brothers. A black eye and split lip might be the quickest way to teach some of these guys the right way to act.
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.
If this is the case, why do these incidents keep happening?
Because as a culture we don't know how to talk about these things - when they happen and when we see and hear about them. And when TV shows - like Freaks and Geeks - come along and try, they're canceled after one season.