I'm a black woman myself who has always pondered on why this is. In the real world, when choosing a 'mate', men often look for a lighter-skinned, or a pale-skinned white woman to marry, simply because light skin is more feminine than dark skin. There are men out there who love dark skinned women, but most men, sadly, do not. The reason why 'light skin= more femininity' is embedded in many men's mind is because light skin is viewed as more innocent and vulnerable. It makes a woman appear easier to take advantage of. To be brutally honest, most men think with their sexual preferences more than anything else when it comes to dating.
I would describe myself as a 'light caramel' skin color, with curly dark brown hair and I find it hard to attract men. I've never had a boyfriend, and rarely any of the guys at my school give me a second glance. They prefer pale skinned white girls with light brown or blonde hair that is long and straight, blue or green eyes, with a small figure because that's just what most men have been found to like. It's an unfortunate fact for us other women out there but remember, no matter how unattractive you feel, there will always be a man out there who finds you attractive. :)