This is an unfair generalization, but true often enough to be useful.

Men fight for social reasons - to show that they're strong, or tough, or to make a point. They often don't really want to be fighting, and once they feel they've made their point, they'll happily back off if they have an excuse to do so without feeling that backing off will make them look bad. Men are aware of, and will follow, the 'rules' of fighting fair, because they know that dirty fighting will make them look bad.

Women prefer not to fight, and will do a lot to avoid getting into a fight. If they feel cornered, however, they will not follow any rules, and will not stop until they have won so decisively that they are sure their opponent is no longer a threat to them. A woman won't just knee your testicles - she'll have that eyeball and never think twice about it.