Why do Muslims consider the side of a woman's head immodest?

What is sexual about the side of your head? In European culture and cultures derived from that Australian, American, etc. only people with severe disfigurements cover their heads. A recent example is the woman who had her face and eyes ripped out by a friend's pet chimp who went nuts. Do you think a woman's head is ugly? And why is it then OK for men to show the sides of their heads? Do we women not become aroused at the sight of a handsome man?