It is the last taboo. Talking about it is not something a nice girl does in mixed company, it is indelicate, unfeminine. Many women have been raised to believe that men are "naturally good" at money matters and women are "naturally bad" at money matters. It's not said directly, little girls pick up this idea by osmosis. Outside the home, for a man to say he wants more money or ask for a raise is acceptable; it goes with the hairy chest and the company car. But women can't say that, they won't even admit it to themselves and they don't want to think about why.