How does society define what's attractive?
I think there are two main factors--genetics and economics. The genetic factor is pretty simple. It pertains to inherent qualities that indicate good potential for procreation. The economic factor is more complicated. It pertains to variable qualities that indicate a high social status. For example, being plump has been considered attractive in some societies because it was supposed to show that you were well off. In our society it's not considered attractive because, I would speculate, it's supposed to show that you're indolent. We're rich enough to assume everyone is fed, so now we're more interested in who's competitive.
Media images of women show the emphasis on competition. Supermodels are uniformly muscular and mannish-looking. Give Kathy Ireland a haircut, and she could pass for Rob Lowe. But beyond that, the intolerance of blemishes shows how economic factors largely govern our evaluation of beauty. Models look like money was spent on them. Their hair is just so. Their clothes are the most expensive. Their every imperfection is covered with make-up. Any problem that remains is assigned to a team of electronic air brush artists who spend valuable time smoothing it over. I would argue that the time and dollars spent are more significant than the aesthetic quality of the end result, which in some cases is questionable, if not bizarre. On some level, we don't think women who spend money on their appearance are hot because of the way they look. We think they're hot because they look like they spent money on their appearance.
Moreover, the least attractive thing a female celebrity can do is neglect her career and play a motherly role. See, for example, Britney Spears. She's pretty plain. The girl who used to work at the convenience store in my neighborhood was hotter. Britney was idolized because someone spent money to make her look (and sound) like something we should notice. But when she started having kids, the men and boys of America convulsed in a collective hurl. Some expressed it in terms of "OMG, what's she doing to her career?" Others expressed it in terms of "OMG, what's happening to her looks?" It was all the same sentiment, though. She was no longer the superwoman who could go out and kick ass in the real world, then come home with a load of cash and kick ass in bed. She was just an ordinary girl with mediocre clothes and probable stretch marks. What could be more disgusting? She almost looked like you'd have to take care of her.
Not that women necessarily have to be taken care of or that there's anything wrong with their having careers. It's possible to be a professional and a mother, and it's also okay not to be a mother at all. But men presumably have careers, too. So, since when do we take only the most intense symbols of female independence as the standard of beauty? Maybe it's because many of us are children of divorced parents and are looking for someone strong to take care of us. Maybe it's because the single-income family is no longer sustainable for most people and we have to look for a capable partner. In the most pessimistic view, maybe we've become so materialistic that the economic factor outweighs everything else.