Women are simply more consultative beings than men are. For whatever reason, they are more timid about making bold statements and deviating from normalcy. They can't shop for clothes alone, they can't judge men alone, and they can't even urinate alone. They support each other through groupthink. Because of this, the average woman's musical taste is more bland and typical. They don't want to be "on the outside".
"But wait," you might say "not all girls listen to pop music exclusively." This is correct. Some women do indeed venture outside of society's musical comfort zone, but their reasons for doing so are rarely musically-motivated. These women are simply looking for a scene or subculture to wedge themselves into, using music as another fashion tool. It's all about image. How many "artsy" girls do you know who insist that their taste is "atypical", "unique", or "eclectic" who have an iPod full of the same girl-friendly, Pitchfork-approved artists like Radiohead, Sigur Ros, and the like? Quite a few, I'd wager. Women are generally considered to have bad taste because they lack any true interest in musical discovery.
Want further proof? How many girls do you know who are into Noise? Who are into Krautrock? Rock in Opposition? Classical avant-garde? Not many. This is because these styles of music, while unique, cutting-edge, and extremely rewarding, have no fashion culture built around them, and no cute boys who listen to and perform these types of music. Music is another article of clothing for women, and it makes basement-dwelling experts froth at the mouth whenever females claim otherwise.