Because it's modern politics and things like these win and lose you votes. You're not offended by it which is great, but others are. And you're deluding yourself if you think that Cameron takes this lightly too - patronising a woman over being corrected on a point of fact. Not good.
Well the woman it was aimed at hasn't batted an eye lid - its all the men that have. If you don't have thick skin then politics isn't the place to be frankly. An overzealous response from male politicians if frankly just vote collecting. Ahhh isn't he nice to stand up for the poor woman (who doesn't really want to be stood up for).....